Extreme Pinball

Go to the option where you select the number of balls.
Press [B]+[A]+[L]+[Enter]. You get 13 balls.

Other fun keys to press while playing:
[PrintScreen] : Capture screen to a pcx
[1] : Turn off anti-gravity
[2] : Turn on anti-gravity
[F12] : Change Scoreboard color
[F1] : Change ball type
[F9][F10][F11] : Different sound effects (Information from Cheat Machine)