Tends to be 2-6-10 or N (Infinite) Key rollover. Lots of membrane keyboards only allow one or two keys pressed in certain groups. "Gamer" Membrane keyboards just tend to make sure that keys most people use like wasd are on different groups (why membrane traces are all over the place). There are other things like bandwidth to take in to consideration as well. So some mechanical keyboards will have a 6 key or 10 key when wireless, but Nkey when plugged in.
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Test above is with a Corsair K68
First two are 8 presses with main fingers and the last one with two fists into the center of the keyboard. The last one is me hitting 19keys at once. So my keyboard has N-Key roll over.