I think that it really depends on the goals of your game and how you're managing character development (stat growth, earning new skills, equipment, class changes, etc) Consider the following:
Does your combat system allow for many playable characters to feel and play different? (Is there skill overlap, skill resource the same, "strictly better" skills?)
Is there going to be a compelling reason for the player to swap characters? (Story driven absence, strengths against certain types of enemies, etc)
Are you're characters thematically varied enough to instill favoritism? (i.e. do you have a bunch of generic humans in different color outfits or do u have different races, unique sprites, battle animations, etc? Are you giving the player fun varied visual styles to choose from?)
I think once you've answered those questions you have what you need to determine an appropriate party size and whether you want to have some on the bench. There are games like Suikoden which have tons of characters to use, with design choices that help them feel unique (unique attacks when allied with specific others, strong thematic differences, formation requirements). Then you have games like FF7 which provide little difference in combat potency between characters and encourage party swaps near completely on thematics.
A small party that is fun to customize can be just as fun as a massive party. Just be sure that if you add additional characters that you aren't doing so just for the sake of adding them. They all need to feel real, feel different from each other, have dialogue, motives, etc. If they don't, absent of heavy player driven character customization (think Final Fantasy Tactics), they are just going to all feel the same and there won't be anything fun about using different characters (meaning that you've wasted a time on that feature with poor returns in "game fun").
Hope that helps! Good luck!