IMHO, any round that will reliably stop a bad guy will also penetrate multiple walls, but to many people's surprise, most rifles and hand guns penetrate more walls than buck shot. If you value your life and your families' lives more than that of a bad guy, don't use bird shot for home defense in any dwelling, therefore #4 buck shot would be your smallest, least penetrating option. Practice, preparation, practice, practice and preparation are all very important. I live in a duplex, and I use #3 buck shot in my 20 gauge close to my bed (I keep my 12 gauge in another section of the house loaded with #1 buck shot). It's readily available at most stores that sell ammunition. I don't suggest attempting to clear any rooms, unless you have to get to a loved one in another room. Get insurance, and let the police handle that. If you do have to get to a loved one in another room, a pistol is more easily maneuverable, but in a shoot out a shotgun w/ buck shot will end the fight the quickest (assuming you do your part--center of mass). However, practice and preparation are key with any weapon you decide to go w/, because the best weapon to use is the one that you are the most efficient with. Oh ya don't forget to practice and be well prepared!
Check out these links for penetration tests with different guns and loads