In my opinion, a standard guy has five fingers, so he can efficiently use no more than five buttons.
Moreover, if he wants to be able to move the mouse, he needs two fingers dedicated to mouse moving, so only three fingers are available for buttons.
Of course, this statement does not apply to fps shooting games nor to space-rockets adventure games, where you keep the mouse with two hands for more precision, and then you have ten fingers available, then you can devote each finger to each of the weapons you acquired. You needed so many time for earning the money in order to acquire that weapon, you have plenty of time for deciding which finger will use this new weapon and which movement will better optimize your placement for using it.
So, my definite opinion is :
For stand one-hand mouse users, a two or three button mouse is enough.
For two-hands mouse users, up to ten buttons can be managed. But you will lose time when switching from mouse-handling to keyboard-typing. So, if ten buttons is not enough for your game, switching from mouse to keyboard will be really time-consuming and you will suffer a tremendous lack of performance.
mind you i hated that mouse... i quite often hit the back/forward buttons which sits on side of the mouse just above where the thumb sits (right-handed)