This works for most tables, it depends on the manufacturer, there are a few threads at VPF on this matter if you use their search....
Press the End key to open coin door.
Press the top 0 (zero) key twice to enter menu.
Press the top 9 key (not numpad keys) until you see
STANDARD ADJ
Press the 0 key to select STANDARD ADJ
Press the 0 key to highlight the number for BALLS PER GAME.
The number will blink when highlighted.
Press the 9 key to increase the number of balls, or the 8 key to decrease the number of balls. Then press the 0 key and wait for confirmation.
Then press the 7 key repeatedly until you are back in PLAY MODE. Press the End key to close the Coin Door.
Here is a thread at VPF that is very helpful in adjusting ROM settings, you can use it's logic to change about anything on all tables.
http://www.vpforums.com/forum/showthread.php?t=42264