A Christian libertarian just argued that atheists must end up worshipping government, and suggested that there were maybe a dozen atheist libertarians. Curiously, he invoked the sacred name of social science, although he didn't actually show any work.
So, it's an interesting question. How many combination libertarians/atheists are there? In the US there are probably 1.2 - 3.2 million of them.
Here are the numbers from easily Googleable data sources. There are around 260 million US adults, with around 7% being "consistent" libertarians according to the Public Religion Research Institute. (other polls suggest around 10% libertarians, with around 15% have libertarian tendencies, but I'm going to be conservative with the numbers).
That gives us roughly 18 million consistent libertarians in the US.
About 27% of those libertarians, 4.8 million, classify themselves as religiously unaffiliated. That is, obviously, a broader category than atheist, including agnostics and perhaps people who generally believe in some kind of God or gods but don't personalize it or act on it.
According to Wiki, between 6% and 15% of the public are non-religious, and agnostics and atheists are about 4% each.
So somewhere between 1/4 and 2/3 of the non-religious are atheists. "religiously unaffiliated" and "non-religious" are not synonyms, but as far as I can find from a quick Google search that's about as good as we have right now.
Assuming, then, that roughly 1/4 to 2/3 of those religiously unaffiliated libertarians identify as atheists, there are somewhere between 1.2 million and 3.2 million atheist libertarians in America. A minority, to be sure, but a few more than a dozen. Even at the low end that's more physicians than there are in the US., and at least as many lawyers. It's enough people that if they founded and all moved to Athelibertopolous it would be somewhere between the 3rd and 10th largest American city.
Of course 3 million people CAN be wrong, so that in itself isn't proof that libertarianism and atheism really go together. But it is pretty strong evidence that it's not hard to avoid substituting state-worship for God-worship.