Tax question

I fully understand that selling crypto in the US gets treated as a traditional investment. Either short term or long term depending on amount of time owned.

What happens if you were to sell all your crypto at one time, but have bought at multiple different price points? If I've collectively spent $1000 and it's worth $2000 at sales, is it just the difference? Or are there other things that happen because I spent $100 at one time, and then $900 at a other time? So both those points in time would have differences in percentage growth? Or am I over thinking that?

Also, for some coins that I've had for years, what if I'm unable to provide proof of purchase? What happens then?

Example above is hypothetical btw