I think 3-4 is absolute optimum. 5 is tops reasonable. 6 is doable but difficult for everyone for lots of reasons. More than 6 makes you a bad DM you cannot possibly keep track of everyone and give everyone the best experience and everyone at the table will suffer because of it.
Not true, this totally depends on how well 'behaved' your players are, and how good your time management as a GM is.
I run a game (shadowrun and das Schwarze Auge) with 7 players, and everyone gets his time to do his stuff.
Not everyone will have HIS shining moment at that particular evening, but everyone has enough time to do his stuff.
Also it helps if you break down the groups into pairs and let 2 of them have do something for about 3 - 5 minutes, and then switch the group (i usually do about 5 - 10 minutes per group, and shorter when it comes to a fight)
The players not involved usually need to check some rules, get something to drink (we play online via skype/discord)
Yes of course you can have more time per player when in a smaller group, that is true. but you are not a bad GM if the group is bigger.
That just means that EVERYONE has to step down a bit and have a bit patience.
But you can orchestrate a much bigger orchestra with bigger /more satsifying results (already ran smaller groups too, but the fights and outcomes were always 'small' with bigger groups you can get to epic with some build up, and my players love it)
I don't need a fancy signature quote from some dude, everything i say is pure gold! ~ myself