usb

HID

Most keyboards actually support more, but cheap keyboards only implement the BIOS compatible subset, which is 6 key supported.

https://www.devever.net/~hl/usbnkro
The USD HID is highly generic, uses a Report Descriptor that tells you the structure of the blob sent alongside it
However, a keyboard can still implement the boot protocol and also provide NKRO. When a keyboard is powered, as per the HID specification, it defaults to using its own preferred reporting formats and can use NKRO-enabled input reports…
A common myth about USB keyboards is that they can only support 6-key rollover, and not report more than 6 keys being pressed at a time. In fact, this is wholly untrue; the USB HID device class can support full N-key rollover (NKRO).