How many American Indian tribes/nations existed in Pre-Columbian times? (references appreciated)?

I realize this probably depends on the specific time-frame, but mainly what I'm looking for is how many nations or tribes existed in North and/or South America at their peak, before Columbus landed in the "New World". I would also appreciate names of the most advanced tribes (such as the Aztecs, the Incas, etc). I'm having a heck of a time finding anything out online. References would be greatly appreciated.