How many of them are vertebrates? Better yet, how many vertebrates are headless?
"how many vertebrates are headless" is the same as "how many headless are vertebrates"
Maybe you meant "what percentage of" -- that could be different because of the different populations.
Of course, we all know that there are no headless vertebrates. In fact, if we were told a life form was headless, we would assert that it wasn't a vertebrate, and if someone showed us bones that suggested it WAS a vertebrate, but assured us there was no head, we would assume either the bones did not go together the way we thought, or the person was wrong about there not being heads.