“Humans sleep 7 hours each night, if that”.
“If that” is a way of emphasizing a small amount or number. So that sentence is a way I would tell you that I don’t think people sleep very much, and probably not enough.
“There were 10 people at the party, if that”. This means there were probably less than 10 people at the party and it was much smaller than anyone expected.
10 people at a party sounds like a lot to me. 10 people means 20 feet, and you guys aren’t careful where you walk. I have a tail and I stay on the floor.