I’m not understanding your question??
The claim is that 388,000 slaves were imported in 300 plus years... That's 1000 plus slaves a year. Where are the other millions of black folks coming from?
A population can't strive in a controlled environment by slave owners. They picked who was going to reproduce and got rid of those that were rebellious. So that means the millions of black population was coming from black indeginous tribe already here.