That's not fixed at all, that's an opinion; the dictionary (several) says either is fine.

There is no final, undisputed arbiter of what is right and proper in English, no Academy of English, no written law. Maybe there is consensus within a social group, nation, or profession, but a whole lot of these things are not provable.

Less/fewer is another one where people pipe up and claim one is "correct" (again, according to whom?), when in actual fact, either is fine and no-one cares and it's not provable either way.