00 buckshot! If there is 2 assalents 3 or so ft apart you'll get them both!
Heaven preserve us from shotgun mythology. Next, someone will be saying that "with a shotgun, ya don't gotta even aim."
At usual home defense ranges, shotgun patterns won't be much bigger than saucer size, IF they even get that big. 'Bore size' is much more likely. Personally, if I have any choice in the matter, I won't use a barrel/load that produces four foot patterns at 75 feet (25 yards), much less a lot closer.
...although I would most likely try to remove their leg at the knee with the slug if it comes to it
And likewise, Heaven preserve us from the "shoot them just a little bit" approach to self defense. Lookit- a shotgun, and ANYTHING fired from it, is a deadly weapon. Its use constitutes the application of lethal force. Any time application of lethal force is not legally justified, use of a shotgun- no matter what it is loaded with- is not going to be justified. I am not a lawyer, and I don't know case law or blackletter law in all jurisdictions, but I'd sure hate to have to go to court with a defense that 'I only meant to shoot him a little bit.'
If you're justified in shooting, shoot for center mass and shoot to stop. Otherwise, find a different way to solve your problem than a shotgun. But no matter what, know the law regarding self defense where you live. And get your info from a local criminal defense attorney, not off the Internet.