I have a 5 and a half month old Lab. So Far he has lost 6 teeth that I know of. Is he losing teeth to quickly, or does that sound about right?

He is a Silver Lab, 35 Pounds. He eats Purina One, large breed Puppy Twice a day. We do not Rough house with him to rough. He has been to the Vet regularly, and Checks out to be Healthy. I am just curious if dogs lose their teeth that quickly.