Warning: contains spoilers for A.X.E: Judgement Day #2!
The X-Men of the Marvel Universe defend the mutant population of Earth - and now readers finally understand exactly how many mutants remain on the planet. Unfortunately, a brutal war is destroying all the X-Men hold dear, including their island paradise of Krakoa. A.X.E: Judgement Day #2 reveals the true number of mutants on Earth - and other planets - but their numbers can be drastically reduced in a single moment.
In current comics continuity, the famous Krakoa reboot of the X-Men series dominates canon and re-imagines the mutant team in a major way. The island paradise, founded by Professor X and Magneto working together, is a haven for mutants (and no humans are allowed on the island without mutant permission). Krakoa is the home of many advanced technologies, including the ability to resurrect mutants back from the dead, and this and other developments have angered humanity - and their self-proclaimed protectors, the Eternals. Determining mutants to be "excess deviation", the Eternals attack Krakoa and the mutants of Earth as a whole.
Thus begins the Judgement Day crossover event, in which the mutants fight a bitter war with the Eternals while the Avengers desperately attempt to keep the peace. A.X.E: Judgement Day #2, written by Keiron Gillen with art by Valerio Schiti, depicts an interesting conversation involving Moira telling the reader exactly how many X-Men inhabit Krakoa. "Krakoa has a population of 200,000 mutants whose average gift is analogous to mildly hallucinogenic body odor." Combine this number with the one million mutants living on Arraki (Mars), the total estimated mutant population is roughly 1,200,000 - but that's only counting living mutants.
The power of mutant resurrection allows long-dead mutants to return from the dead, even years after they fell. There are over fifteen million mutants who are currently deceased, and potentially all of them can come back. As the Resurrection Queue is over fifteen million mutants long, it is the Quiet Council's eventual plan to resurrect every mutant who has ever died throughout history. If humans learned about this endeavor, they would be even more incensed than they are now (and the Eternals, already fighting the X-Men, would stop at nothing to end mutant resurrection for good).
The Resurrection Protocols determine exactly which mutants are brought back before others, and which ones cannot be brought back at all (precognitives, clones, and others determined to be a threat to Krakoa). The mutant population of the Marvel Universe is often subject to one disastrous plunge after another (M-Day courtesy of the Scarlet Witch, the disaster at Genosha, etc). Thanks to the X-Men and their new era, however, any death suffered by the mutants will eventually be undone.