Are there always openings for combat engineer's in the army?

idk how this works, but i want to go in as 21B combat engineer but i read somewhere that i wouldn't be able to if there weren't any openings, how does this work? do i tell my recruiter thats what i want(haven't talked to one yet) and not quit til i get the mos i want? and are there any medical problems that could stop me from getting it? like eyesight or something like that?