6 Keys on USB, if I recall correctly. However, PS2 doesn't limit you at all, and a number of keyboard let you press any number of keys you want. (Typically high-end mechanical keyboards, or gaming keyboards.) A honza mention in his (dead) comment, the technical term is "n-key rollover" where n=6 for USB, and n="n" when it's unlimited.
The newer USB keyboards with n-key rollover don't have this limit. I'm able to depress 22 keys at a time on a Sidewinder X4 ($45) or a Filco Majestouch ($120) connected via USB and have them all register perfectly. Yep, the steno layout is only two rows for fingers, one row for thumbs, and one meta-row for numbers. There's very little movement of the hands required, which makes it both ergonomic and efficient. Chords of 1 to 22 keys are mapped to syllables, words, or phrases. It's a lot like playing a piano.
