There is no magic number really. As long as you can make the characters skills diverse enough that they aren't just clones of each other, and if it's a story-driven game make the characters interesting enough to care to try them there is no limit. It would also depend on how your using them. is the player choosing them out of one big pool? Can the player rotate characters in and out? How long is the game? Is this the sort of game that takes a few hours to play through with emphasis on trying to complete it with different characters? If it's longer game, I would strongly recommend the player having a lot of freedom in switching them out. Characters on the bench should also gain some XP or leveling the party is going to be extremely tedious.
In my game, there's are 10 playable characters in the party. The battle party is the default four. The four that finish the battle get full XP, and the other 6 get 1/2 for the above reason. My characters have a fair amount of classes, which bring up another question. Are your characters locked into one class or are there several to choose from? If it's the latter, I'd say be careful about how many characters you have then. Having lots of options is great, but that can get out of hand quickly both from a development perspective, and from the players feeling overwhelmed.
One of my favorite games with a huge character pool is X-Men Legends II. For every mission, you chose four characters out a huge pool. Every character had a single class. Stat and skill points were earned on Level up. You could freely choose which stats and skill to improve on each character. Characters that you don't use for a mission still got some XP (not sure if it was 50%.) Equipping items was pretty simple. Too lazy to check my lown link and see how many items you could equip :/ I'm pretty sure it was just 2 or 3. If you have a lot of characters you may want to have a lower number of equippable items, but it can be done with more as long as you're willing to put in the work.
Hope that helps
Large pool you can swap out freely, that was very important to me. At any given time, the main plot progression will dictate active party (sometimes they are split up, and you get no control over how the plot splits them up), so at times you have to use this character or that character.
It's going to be a very long game. I plan on the "End" actually being more game. A free-roam portion of prep time, and then a new mission that will take you to a new area, an impossibly large tower, every floor having it's own landscapes. You wil have to scale this tower with the entire party doing bonus quests, with bonus story, script, battles, and bosses.
The shared exp thing is a nice idea. I fear the ability to lvl up X amount of characters without even using them . Perhaps I'll lower the amount gained to 25% or less. I plan to make my characters extremely valuable each in their own right, and I can this easier and better with less, so I'm going to trim some of them to secondary characters, that may never even enter the party, or briefly for a portion of the story before leaving he group.
EDIT: They are also limited to 1 class per character, but I have added ways to customize and tweak builds using skill books and stat boosts (I'm regulating thet also, so it can't get too out of control. Ingredients to craft boosts themselves will be extremely rare, then you have to create them).
Edited by Rio Cortez, 13 February 2017 - 05:28 PM.