"Adherents.com is a growing collection of over 43,870 adherent statistics and religious geography citations: references to published membership/adherent statistics and congregation statistics for over 4,200 religions, churches, denominations, religious bodies, faith groups, tribes, cultures, movements, ultimate concerns, etc. The religions of the world are enumerated here." [there are a lot of ads but useful info if you drill down]
Adherent as defined by Merriam-Webster:
a : a follower of a leader, party or profession
b : a believer in or advocate especially of a particular idea or church
NationMaster - religion statistics by country.
World Christian Data - "provides comprehensive statistical information on world religions, Christian demoninations and people groups.
International Religious Freedom Report - "published annually by the US Dept. of State, this report describes the status of religious freedom in every country. It looks at government policies violating religious belief and practices of groups, religious denominations and individuals and U.S. policies to promote religious freedom in the world."