Everyone loves Aerie.

The absolutely ancient Wizard's Crown, IIRC had 8 or 10 to a party, all player-created (can't remember, this has been many years since I've played that). But, the tradition generally runs 4 or 6 characters, sometimes with slots for NPCs to join later.

In a game like you've been describing yours to be, a few dedicated slots for such NPCs may be a must. Also, consider if your mages/casters have access to permanent summoning spells (like in Bard's Tale).

Interesting question, one I haven't considered much since I knew I'd go with tradition on this one. Still, you can't go wrong with offering 10 slots total; a few to account for NPCs that are required to join at various times (probably 2 will be fine, but whatever may be needed) and the rest player-made. Of course, if the entire party is NPCs, like in BG, save the main character, then 6 or 8 may be fine there. Though, if you have a lot of options for character classes, then offering room to employ more of them at once may be good.

It's kind of a hard question, as we don't know exactly how in depth the battles might become. Only you, and a few others, can really answer that. A huge party may be necessary if you have battles in the scope of Age of Wonders, which would have upwards of 56 units in a single battle. If it's more like many of the fights in say, Treasures of the Savage Frontier or Ultima 6, then smaller parties should work fine.