Most PS/2 keyboards do not have N-key rollover. The majority of PS/2 boards are limited to 2-key rollover since they're membrane designs. (when describing rollover or the number of keys you can simultaneously press and register, you generally use the lowest possible rollover.)
However, N-key rollover is possible on PS/2 if your keyboard supports it, which generally requires either capacitive switches or mechanical switches with diodes under every key. If you have a board that supports N-key rollover, you can hold down all 104 keys and have them all register.
USB by comparison is limited to 6 keys at once plus a few modifiers (Shift, Ctrl, Alt, etc.) Most USB keyboards again are 2-key rollover due to matrix limitations.http://www.microsoft.com/appliedsciences/AntiGhostingExplained.mspx