A new genetic study finds that the original Native Americans numbered between 229 and 300, with an average estimate of around 250.
Note that that is an "effective population," which means the number who contributed to the eventual gene pool. Because some of the founders must have died without leaving descendants, or lost all their children, the actual population must have been larger than that. Statisticians argue about the right figure, but nobody thinks it should be more than double the effective population. So if this calculation is right, the vast majority of American Indians in 1492 descended from between 229 and 500 first settlers.
That's actually a pretty big band; I would have thought no more than 150 or so could hang together through the crossing. But 229 to 500 is what the genetic math is saying now.