As Illia has pointed out, the rareness of a breed is dependent on the criteria used in doing a census for them.
Do we actually count Easter Eggers? The reason I bring this up is that SO SO SO SO SO SO many people assume they have Ameraucanas, when in fact they have the EE mutts. So, who does the counting there?
Then you have to ask, how is the information scrutinized? So, someone says, "I am a breeder" but they don't have a good containment system for keeping lines pure and you get crosses thrown in with your pure stock? There are some places, I could name one in particular even, that deals completely in 'rare and heritage' breeds, but they produce a lot of junk.
Plus, some so-called breeds, are here in the US simply because someone thought they looked cute in their home country, brought them here and now we have a new 'breed', when in fact, they are a mutt type breed from another country that follow no standard. This, to me, is just a foreign Heinz 57 group and not necessarily a breed.
Then there are numerous breeds that aren't even recognized by the APA, so how do you draw the line? Brabanters are rare here, but they are a fairly recent import. Do we really need to count them? How is there population in Europe? There are others too.
So, back to the point, depends on your criteria.