Why so many denominations? Broadly speaking, lack of highly centralized authority, otherwise there’s a wide variety of reasons, some sectarian but mostly non-sectarian and/or clerical. Well, I like to call them clerical, but that’s not important right now. Much of it has to do with self-government, although self-government is almost never synonymous with a claim to a tiny monopoly on all truth, any more than every single denomination is synonymous with disagreement and sectarianism. Bottom line, there are good ways to multiply denominations and there are bad/sectarian ways. There are also silly ways that don’t mean anything, and it is all of those things put together. Prevalence of one over any other one varies from time to time and place to place.
My main source is going to be the US State Department. These numbers are from '06, so bear in mind that a half-million semi-lapsed Catholics joined Eastern Orthodoxy since then, along with approximately 800,000 more (nearly all impoverished Mayans in both groups) that came into Oriental Orthodoxy with Antioch. So Orthodoxy as a whole is now around 9% and Catholicism is no longer close to 60%.
Www.state.gov/j/dlr/rls/irf/2006/71462.htm
“Although there was no accurate census of religious affiliation, some sources estimated…”
Granted, that is vague, but it’s the best that the US State Department could do. So that’s what we’re working with.