Regardless of the game played, 4 is optimum, with 3 and 5 also working quite well, but everything else not.
6 is somehow the magic number at which group dynamics kick in and the brain triggers the reflex to let two or three people take charge and just go along with that. If all players are of a very active type that wants to provide input on all descisions, 5 people still works. If there are one or two players who don't push forward to get themselves heard, even that is too many. Having two highly active players and two quiet ones is a bit difficult, but otherwise 4 works very well. If you have two very active players and one quiet one, 3 is a small enough group where that doesn't matter and the others will ask the quiet one for input.
However, I once played in a Shadowrun game that had 7 PCs and 10 people in a tiny kitchen, that was a blast. Because most of the game was bulshitting about the incedible plan we came up with to investigate a warehous, with everyone having ideas how their characters could contribute.My sniper got to the othershore of the river to give fire support from the riverside, where there was no wall and I had open sight on almost the entire yard and the insde of the building.
And then of course, being Shadowrun, we completely




ed it up. ^^
Shadowrun works well with large groups because it's a genre that works a lot with large teams of specialists doing there thing in a very condensed timeframe. Getting the contract, scouting the location, planning the mission, and executing it was the entire adventure. Nothing with talking to NPC, getting to learn the background of the world, dealing with the landscape, and what you all get in many other RPGs.