USB keyboards have a maximum of 6 simultaneous keypresses (plus modifier keys -- Ctrl, Alt and Shift).
Last I heard, in USB mode, the IPac has a maximum of 16 simultaneous keypresses (plus modifier keys).
USB gamepads have a maximum of 64 simultaneous inputs.
NOTE: The above is my recollection based on the "Great USB vs PS/2 and Keyboard vs Gamepad ThreadWars of 2005"(tm)
For more info, I refer the ambitious reader to
TigerHeli's Arcade Interfaces Page, which includes technical discussions by the major participants of said ThreadWars.