The source of that information would be the same for us as for you -- Google is your friend. You'll find however that the numbers are little more than educated guesses, as often the question isn't asked, or atheists, agnostics and/or the unaffiliated / non-practicing religious are lumped together.

There are, doubtless, millions of us in the US alone, but then we come to the fact that how many of us there are is no signifier that we're right or wrong to withhold belief in deities. That would have to do with the reasoning behind atheism, not how [un]popular it is.

Hi welcome to the community.

Ask god I believe he would know. He has a list of people not on talking terms with him, REmember this is worse than not being on Santa's List