I’d say 4 is ideal. At that point, someone will be on the bench during a round in a conflict, which makes for interesting strategic choices.
I have seen tables of up to 10 but past 5 you have problems getting everyone a chance at the action. What can help is relying on the party leader to pull the group together, and letting them table chatter for who does what. When the party decides on their action, I go around the room and see who helps. If someone doesn’t help (or can’t) that’s a good opportunity to ask, ‘what are you doing in the mean time?’ If that person hasn’t had much spotlight, you can present them with the next obstacle and get them rolling the next turn (perhaps triggering the grind).
Large parties also can throw a lot of dice with help, so it’s good to be a little stingy with what helps. You might even say something like “only 4 people can help on this roll. The rest can help by creating a skill supply.” This can spread the rolls out and allow for some obstacle management. Throw in an evil gm factor if it still seems too easy for a large group.