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.