Sperm counts?

Just wondering what’s considered low sperm count? Our RE really hasn’t said but I’m concerned. We are unexplained. My husband has had several done because we’ve had several IUI’s. His count is anywhere from 9- 25 million post wash. Volume is usually around 1 - 1.5 ml. Our Dr said that the volume could just be normal for him. Apparently no issues as far as motility or morphology or at least it’s not been brought up to us.