My 17 mth old has 5 teeth completely through and 2 just coming through. I'm not worried, someone told me it runs in family's, and my partner was very late with his teeth as a baby. But who knows. It's definitely within the normal range, so don't worry.
My 20 month old has 12 teeth. But I don't understand how your teeth not coming in yet could be considered 'behind'? It's not some developmental milestone. They're just teeth. DD could still eat loads of things when she only had one tooth, though I agree having more may make it easier to chew.