... how many ribs?

They're manly ribs with a female coating.

and what exactly do blastbeats have to do with onanism?

blastbeat is the artist name; he doesn't seem to have a tag on danbooru yet

the artist profile mentions that he is a drummer and a fan of death metal

Too bad Japanese death metal sucks. :(

I was referring more to the fact that if you imagine her skirt is not there, you'll realize her torso is about twice as long as it should be.

can't unsee ;_;