There will be a limit, but I'm not sure how many. I've always liked smaller parties (4 max) since those are more in line of the adventurer groups you'd see in JRPGs, lore tales, etc. You know, your four characters for a balanced party. It might be upwards into 8 or 10 though...which I personally don't care for.
Like four people going to a dungeon or cave: plenty of danger and fun to be had. Eight or ten going in, there's no danger of anyone possibly being killed, everything dies. Take Lord of the Rings...you had your melee, your ranged fighter, your wizard, and your tank (no healer unless you count Gandalf)...and the tag a longs (them Hobbits) while important to the story, don't really count towards a battle party.