Press some keys simultaneously to test your keyboard for blocks done by its matrix (has your keyboard "n-key rollover").
Different keyboards have different topology of matrix, so there is no unified method how to define the limiting number of keys.
Try find out a blocking combination which consists of the lesser amount of simultaneously pressed keys.
Keyboards with a matrix do guarantee only 2 simultaneously pressed keys to be scanned correctly.
A properly done keyboards have no any limit at all.
Note that USB may apply an additional limit of 6 (or 18) simultaneous keys, but I never have gone so far to use such amount of keys.
Be also deliberate when dealing with a "gaming keyboards", - those might appear having WASD and cursor keys being excluded from the matrix (having own leads with no sharing), what doesn't mean that all other keys also aren't a part of the matrix.
(WASD by the way is almost the worst configuration for a games, because there remains no useable keys at the left side from the hand; worse configuration is only to play with arrow keys).
If you still don't know what to press, then press and hold UIOP and then try any other buttons; after that, move your tests to the numpad by lets say pressing and holding there 456 and then trying to press some other keys additionally.

Keys received: Count: 0

Another page with the same tests for a keyboard is on http://gadzikowski.com/nkeyrollover.html.
The original of javascript for this test was taken from http://random.xem.us/rollover.html.
Explanation of keyboard matrix problem is at http://ai222.narod.ru/kbd_limit/kbd_limit.html.
More links : http://en.wikipedia.org/wiki/Rollover_(key).
Note for SDL users : when using versions like SDL-1.2.13, they require that Numlock would be on, otherwise after a conflict, SDL will reset the state of some keys. Older SDL versions didn't need that.
Note about PS/2 : the original PS/2 driver of Windows XP does not limit the number of keypresses it can pass; I got 13 on one 2kro keyboard (I have no more thingers to try more keys :p); on another 2kro one I got 6 max (means that keyboard internally limits the number).