Obviously he means whoever. What else do you shoot at? Rocks? (Maybe tires on cars, but Severus has no idea; what he does know is most people shoot at other people, no matter the scale.)

"How much more is there?" he asks instead of saying No like he wants to. Or thinks he wants to. The k-word is a dangerous one - Severus has a hard time truly brushing off learning new things, even if they seem stupid.