So more shot per shell just means more spread? Is it not going to penetrate as much either?
In a given area,
The smaller the shot,
The more shot.
And spread depends a Lot on the choke.
My trap gun has a tight choke,
small spread, more shot in the group.
My Skeet gun has a more open choke,
less shot in the group.
I would suggest #4 birdshot. It will go through muscle, flesh, and bone. I know, I've seen it. In close quarters, like your home, it will be lethal and over penetration won't be an issue. If by some miracle they survive the first shot, then you hit them again.
It takes 10 yards or more for any choke to spread more than a few inches. We have already established there is a one hour fire rated wall between apartments. I seriously doubt there is more than 10 yards inside your apartment. Get yourself a can of pepper spray if you are so frikin paranoid. You will not be able to use a weapon with all these crazy notions.
"I do not aim with my hand; I aim with my eye. He who aims with his hand has forgotten the face of his father.
I do not shoot with my hand; I shoot with my mind. He who shoots with his hand has forgotten the face of his father.
I do not kill with my gun; I kill with my heart. He who kills with his gun has forgotten the face of his father."