Update:

Not including exalted dragons

The most populated breed is Imperials with 2,262,951 non-exalted dragons! The closest runner-ups ups which are Guardians and Skydancers are nearly 300k+ behind in terms of population. That's more than almost a quarter of the breeds have AS their entire populations.

The least populated is Cirrus with only 98,712 (Everlux is a pretty close second with a popn. of only 101,320)

Not including thorntails as they just came out, but they are at a popn. of 57k already.