HOW MANY IS TOO MANY?

I think the swapping system sounds really cool, and honestly I think 2 characters in a battle is fine. especially if you can switch out to other during battle.

Something that might be cool and also help with not abusing the swapping system is maybe more enemies come when you swap(if that's possible to even code) just that way you don't have 11 people to swap out with, without some repercussion.

Think of Final Fantasy 10, if you have ever played, where you needed some characters to fight certain monsters and you could switch out mid-battle.
that's something to consider too.

McBick: Consider not using perfect 'elemental pairings' (Fire element dude can be weak to Wind instead of Water) to make the decision process behind choosing an ally more interesting. Otherwise I'll always use the pokemon ally who has the elemental advantage, every single time. Another form of averting this is giving the Fire dude who is weak to Water some attacks that do special things to Water enemies, such that you might actually want to think about taking a gamble and leaving him in.

Right, that's exactly what I am talking about. When you go up against enemies who can only be Ignited, you only have one choice of which ally to use. I'm not sure why it's interesting to switch party members around if it's always the same party member being applied to the same situation every time...

(This should be in game design and theory, probably, and is actually a pretty interesting discussion.)

I personally think that two allies is *usually* too few to allow much depth to combat tactics. You don't have enough people to let different characters really perform different roles. However, the fact that you can switch them out mid-battle helps a lot - especially if doing so doesn't take an entire round. (Is it completely free, or is there some sort of MP cost or other cost? I don't think it should be completely free, but taking a round is too steep a cost.)

Might I suggest an idea I got from the Roguelike genre of RPGs? Many Roguelikes let you start with one ally in the way you're describing, but as your charisma stat increases, you can use more allies. You don't have to actually base this on the player's charisma stat (or have a charisma stat) - you can unlock the ability to use more party members through some sort of story event, key item, unlockable ability, etc. This makes the game start off simpler, and get more complicated as the player gets accustomed to it.

Here's kind of a breakdown of what kind of tactics I think you can generally expect from games with different numbers of party members:
1 party member - Main character must be able to do everything. You almost always can specialize your character's build in this type of game, but you must be able to beat every enemy regardless of how you've built him. This typically means that either he must be a jack-of-all-trades who only gets minor benefits to whatever he specializes in, or enemies must only be slightly harder to beat with the worst strategy than with the best one. This type of RPG is typically easy.
2 party members, locked for entire game - One will be physical and one will be magical. Even if you let the player customize their builds, this will be how every player chooses to build them. Between them, the player will generally have access to every offensive strategy in the game. Almost always, they are both pure damage dealers, with healing done via items or via combo techniques or via some other method that they can both do equally. Occasionally, one may be a tank and the other a healer, but this doesn't leave much room for anyone to deal damage. Having one be a damage dealer and the other be a healer doesn't work very well - it just isn't fun and the healer often feels worthless. Significant interaction of some sort between the two characters, whether that means healing/tanking, combo moves, moves that make other moves more powerful, etc., is guaranteed, and will typically be a major part of the gameplay.
2 party members, one selectable - The hero will be vastly more powerful than the selectable party member, and will be able to effectively deal damage to any enemy. This is closer to a single-character game than to a game with two permanent party members. The selectable party member takes the place of customizing the hero's build. If the members can be changed mid-battle, then you lose the customization in favor of being able to always have access to every skill in the game.
2 party members, both selectable - I've never really seen this done. Good luck. It would most likely involve a large amount of interaction, both between the two active characters, and between the active and inactive characters. If you can change mid-battle, this would be similar to a three-character system with 2 or 3 selectable characters, except with a lot less room for player error. If you can only change outside of battle, this would be difficult to make work, since the player would usually (but not necessarily always) be limited to combinations that satisfy the stipulations of the "two permanent characters" situation.
3 party members, locked for entire game - You no longer have a balance between magic and physical, so instead the three members are typically a tank, a healer, and a damage dealer. In some games they may include a healer and two damage dealers. The fact that the healer has multiple people to heal makes his actions feel more tactical/interactive and less like a simple HP battery, and creates a good balance between single-target and area healing, where the player actually has to weigh the pros and cons of each in situations where two of the three people are hurt. In other games, all three may be jacks of all trades, differing in weapon types or elements, which results in less interaction between party members.
3 party members, at least 2 of which are selectable - This is a solid setup that allows for a lot of advanced tactics. Even if you have a main character who is unremovable, the hero has enough support that he's no longer required to be a jack of all trades. You can do almost anything here. The player will almost always use the same types of role combinations as you would give them in the previous situation, but must determine which of those combination to use, adding a significant layer of strategy.
4 or 5 party members - The player is absolutely guaranteed to have more than one damage dealer, which opens up a lot of possibilities for complex offensive strategies. The player has enough characters to use multiple healers in situations where they are taking heavy damage - this ability to switch between a more defensive and more offensive focus can lead to people who are good at strategy utterly overpowering battles that you planned to be quite hard, and requires you to make bosses that summon allies or heal or otherwise make sure the player can't win every fight by turtling. Area healing and area attacking becomes extremely effective compared to single-target healing and must be nerfed in some way other than merely MP cost. This is the lowest number of party members where enemies that use instant death spells can be balanced into the type of game, although in a 4-character team they're still extremely dangerous. This type of party has a great deal of complexity - probably the most of any of these.
Lots - At six or more characters the player essentially has an army. Interaction between characters is rare, since anything that can interact with only one character is useless, and anything that can interact with many characters can be used way too often. Redundancy between characters is guaranteed - often signifcant redundancy. One character dying is no longer important to the outcome of the battle. Enemies that use instant death spells become quite acceptable. Area spells become absurdly powerful, and must either only hit a subset of targets, or have extreme limitations on their availability for usage.

Right, I know you get more character options as the game goes on. What I'm suggesting is that when you get your first partner, and for some time after that, you can only use one partner. But around the time you get your fourth or fifth partner, you can start using two partners at a time. And around the time you get your eighth, ninth or tenth partner, you can start using three partners at a time. Or something like that - the exact timing of when you can start using more isn't that important, although you should probably always have at least 2 more partners than party slots.

I think it would make it easier, really. You no longer have to worry so much about making sure no one party member is too optimal, etc. I dunno. I like complexity. I don't think there's such a thing as too much. That's why I make RPGs. Your system as it stands, with only one party member at a time, means there's always a single best choice as to which party member to use. In battles against these enemies with weaknesses, you cannot choose partners for their tactical advantages and side-effects - you must always use the one that matches the enemy's weakness. Giving the player two partners at a time would mean they have an actual choice of who to use, creating a more RPG-like feel and a less simon-says-like feel. But giving them only one partner at a time for the first third of the game would give them the ability to become accustomed to it, and give you time to include plenty of enemies and bosses that only are balanced with one partner.

author=McBick
Of course in any poorly designed rpg there is an optimum strategy to use against each enemy or enemy groups.

Fixed this sentence for you

author=LockeZ
Your system as it stands, with only one party member at a time, means there's always a single best choice as to which party member to use.

While McBick does have a point, that any sufficiently complex system will have a small selection of optimal choices for any given situation, there IS in fact usually more than exactly one optimal choice. I tried to stress this point in my earlier posts, and LockeZ is now repeating it, so I would definitely stew on the switching mechanics for a bit.

No, Jesus, that's the opposite of what we said.

Do you only read every other word?

Okay, so English is not your first language. Got it.

To "stew on" something means to think about it for a while.