Data Sets from Surveys and Statistical Reports can give a great snapshot oF how many in a specific population (or group of populations) may think, act, or affiliate at a certain point of time in a certain place or places.
The World Religion Database contains detailed statistics on religious affiliation for every country of the world. It provides source material, including censuses and surveys, as well as best estimates for every religion for the period 1900 to 2050.
Many denominations now post some of their membership statistics online.
