i think any more than 8 buttons on a mouse is ridiculous. but i find that on mice with a lot of non standard buttons, half of them dont work without installing some bloated software on your comp, and even then most of them never get used. my current logitech bluetooth laser mouse for example, i can use maybe 5 of the 8 buttons in most games, though the middle button is hard to use most of the time. two of them mirror the function of the scroll wheel and the other one doesnt work without installing the software. while the sidescroll works in some applications, its completely unbindable in games. i might open it up and rewire the buttons so that i can use that 3rd thumb button as middle mouse, and the scroll wheel button can be the useless button you cant use without the crappy software.
if i were to design a mouse, id give it a 4 pack of buttons up top (2 each side), about 3 thumb buttons and id eliminate the scroll wheel in exchange for a ministick (which can be configured either as a joystick or 2 axis scroller). that or id go with 2 thumb buttons put the ministich down theire for the thumb, and include a simple scroll wheel. though microsoft has been doing experiments where the top of the mouse is essentially a touchpad with areas that can be sliced up and mapped out as buttons, axes, whatever. while its cute i still think a mouse needs to click.