I currently run two games on alternating Fridays. I run an L5R game that's been going for about 6 years, maybe longer, is down to four players. I also run a D&D 5e game that's been going for about a year with six players. Between the two games I've had something of a personal revelation in regards to what I think my ideal 'party' size is.
For the longest time I didn't really care. The best game I ever ran was like thirteen players and it worked. However, in running the two games I sometimes find myself flustered in L5R that things drag way down if a player isn't sure what they want to do or work on while other players are already locked in. Basically, not enough people are doing things to make time pass, which I feel makes the gmae less than ideal. On the other hand, in the D&D game it is hard to juggle things for everyone and get everyone meaningfully into every session. I know this is impossible, but with active players you sometimes end up with too much going on and it makes things hectic.
Which leaves me thinking that if 4 is too few and 6 is too many, that 5 is probably about right. Obviously the players will influence this. I also imagine other factors will kick in as well, including how long a session can be and the day/time it's happening. My games happen on Friday evenings, but in both games we have people who need to get home for various reasons and so we only get 3-4 hours which feels...confining at times. Perhaps a saturday game that could run for 5 hours would make that ideal number go up - or down - but that's not what I have.
How about you? What's your ideal number of players in a table top game? What's the most you've successfully pulled off that wasn't just a one shot or something else meant to be small and quick?