Are you familiar with this website atheistcensus.com?
It is run voluntarily by Atheist Alliance International (AAI), a non-profit organization. I guess atheistcensus is a bit limited only to depending on who has a computer and email address to be counted in. By observation of data from above website you may see some 237 countries do have people who have computers and email addresses but U.S. leads the world in "Count me in" sign-ups.
If you wish to count in yourself and validate by reply email, be sure inputs are good for permanent record because you cannot update your record. Should be only one email address just to represent you one time.