Prefered RPG Party Size? [on hold]

Prefered RPG Party Size? [on hold]

I'm currently developing an JRPG-style battle system with many simple, but
unique, aspects put together, allowing each and every playable character
to have an important role in battle. One can deal high end physical
damage, another magical damage, a healer, a tank, a member who can easily
stagger enemies, etc. Essentially the usual roles with a few aspects that
allow enough uniqueness between characters for each to play differently,
but enough overlapping that if players were to only use their favorite
ones they would not make the game more difficult for them.
My primary concern is how many people can be used in battle at a time.
I've seen RPGs with anywhere from 2 to 6 party members that can be
fighting at a time. I've also seen games that allow you to switch party
members in and out in the middle of battle, such as Final Fantasy X. Since
I intend to have around 9 to 10 playable characters throughout the game,
this gave me an idea (to use a similar party swap system as FFX), but I'm
still questionable on whether it will restrict players from managing the
active members in such a way that battles will still provide a challenge
without becoming too difficult.
Does anyone have an opinion? Or maybe some advice as to how to resolve
this issue?