How many keys can my keyboard press at once?

Rollover limits follow the physical wiring inside the keyboard, not a single number. One group of keys may happily report six at once while another blocks at three, so test the combinations you actually use rather than a random handful.

When you exceed the limit, either a key you are holding stops being reported or, on older designs, a key you never pressed appears. Both are called ghosting, and neither means the keyboard is broken — it is a limit of how the board scans its key matrix.

The basic USB keyboard protocol carries six keys plus modifiers in a single report, so six is the practical ceiling for any keyboard connected in the standard way. Keyboards advertising full n-key rollover switch to a different reporting mode to get past it.

If yours stops at two or three on ordinary letter keys, that is a low-cost membrane matrix rather than a fault. It rarely matters for typing, because you do not hold six letters down at once, but it matters a great deal for games that expect several movement and action keys together.