I still have some work to do, but it would appear that based on the following criteria:
1) The hardware has to be manufactured or officially licensed by Nintendo. (i.e.: An NES, Wii U, Panasonic Q)
2) The hardware must have a serial number (or was supposed to, in the case of some of the 2nd hand Game Boys - i.e.: not a Pokemon Mini or Game and Watch system).
3) The hardware must (reasonably) play games (i.e.: not the Game Boy Camera/Printer or a Game Boy Player)
If my estimate is correct... I have 70 Nintendo systems. I could seriously build a fort.