Modifier keys are usually specially dealt with on the matrix of wires in your keyboard to make that work. The issue is how many non-modifier keys the keyboard

Modifier keys are usually specially dealt with on the matrix of wires in your keyboard to make that work. The issue is how many non-modifier keys the keyboard can register at once. That said, other comments note that Apple seems to lay a dense matrix that allows for high numbers of simultaneous key presses to be registered even for non-modifier keys. Perhaps this finally explains some of why their keyboards cost $50 instead of $3. (Definitely not all, but if this is true, it's a nice touch.)