I built one of those residental house where NPCs can live too (but not a high rise, they're trailers) and I have like fifeteen neighbors! The thing is, I only have two other trailers, so it must be pretty crowded in there.
I have a dog house near the trailer my sim owns for her four dogs and people keep coming up to them and petting them. Neighborhood dogs!
I think it's weird since usually when I build NPC-livable lots I only have one or two neighbors...here's a picture of some of them (its at night, and they changed into their firefighter clothes here)
I like to call the the 'Lazy Firefighter Gang' since they're always changing into their clothes, but they come back an hour later to go back inside
You can get an idea of how small the trailers are. You can also kind of see the dogs at the right, too. Thats only about half of the neighbors too.