3 kids, 2 parents = 5 iPhones (2 used as iTouch, 2 with data plan, 1 voice only).
Charge and sync is a pain. I tried connecting all five to one Kensington 7 port dome USB hub. It worked ok until the fifth connection, then the newer iPhones (4, 3GS) started complaining that they couldn't charge of an unsupported peripheral.
So four is the practical maximum for this hub. I suspect that's about as good as any hub will do.