Most standard keyboards offer only 2KRO, and although it is possible that certain key combinations allow more keys to register simultaneously, this is not guaranteed, and may differ even between different revisions of the same keyboard. True N-key-Rollover (NKRO) means that all keys can be held down and register correctly. X-key-Rollover (where X is an integer smaller than the number of physical keys) is the minimum number of keys that will register correctly under any circumstances. The keyboard enthusiasts forum at has an extensive list of keyboards (login required) tested for their N-Key-Rollover capabilities, as well as instructions on how to find the number of simultaneous keypresses a certain keyboard supports.
