The absolute highest I've got is 7900, I wanna get up to 8000, but I don't think it's possible.
EDIT: I was wrong, I've gotten 8950 just now, and yes, once again, I've got proof.
It seems like you start out with five aliens, and when you kill them all, the numbers increase by one, so it goes, 5, 6, 7, 8, 9, 10 and so on.
The spear is the shit because it'll kill what's in front, behind, and to the side of you.