There are multiple things going on here. First there's a difference between spoken and written language. Most of the differences between spoken British English and American English. Since spoken language tends towards the informal and Wikipedia is written and more formal.
Wikipedia does have multiple languages (see the left panel for the various ones), but it doesn't have separate American, British, Australian, Irish, Indian, etc etc versions. Scots is considered a distinct language from English though it is very close. There is a separate Scots language Wikipedia but it was notorious for having many articles 'translated' from the English articles using automated one-to-one translations of some vocabulary into Scots.
Wikipedia has an English language policy:
The English Wikipedia prefers no national variety of English over any other.
Anybody can write anything on Wikipedia - so someone might edit spellings to their own local standard, or may do it in one part of an article and not another. The Wikipedia style guide says
American English spelling should not be respelled to British English spelling, and vice versa; for example, both color and colour are acceptable and used in article titles (such as color gel and colour state).
There is nothing in an article that says "This article was written in American (or British) English". But there may be clues in it - spelling choices may be the most reliable ('-or'/'-our', or '-er'/'-re') are common enough that a long enough article may have one.
But there is no easy way to comprehensively judge of all English articles, which variety of English is used by the author, so it is unlikely at the moment to be able to discover exactly what the ratio is. My semi educated guess is that most (>75%) English articles have been written by Americans using American style because 1) Wikipedia was started by Americans, 2) there are 330M USians vs 70M UKians, 3) I don't feel like I see many British spellings on Wikipedia.