Honestly a ton of them died and there weren't that many to begin with, then a lot of them bred with settlers. Honestly I don't think it matters. Pretty much everyone I meet is a mix of many different races. In fact I think it would be kind of weird to meet someone who was like "yep, I'm 100% X!" That's just...really strange.
Yea, no. Historians estimate the native population was between 1 and 18 million (from Wikipedia). 300 million is super wrong.