I definitely think the number you have depends a lot on the time you have available to give to them as well as how much time you like to spend with them. I really like to interact a lot with mine as they enjoy the stimulation and it makes them really confident. I've had six pigs at one time before, but I wouldn't have that many now as I don't have as much spare time so would have to compromise on the amount of time I could spend with them and I'm not really happy with the quick cuddle here and there. I like to spend more time with them, than mainly just feeding them and removing their droppings!
Having them in the living room helps as whenever I watch tv, or have people round, they're right in the middle of the action, running around or handy for a cuddle. It's the room we relax in, so it's nice having them right there.
I think when introducing a new pig, you have to think about harmony as well. If you have a pair that get along really well, introducing a new pig could upset things at least temporarily but maybe permanently. Not just in terms of the new pig not integrating, but also in terms of potentially affecting Furlong and Ascot's relationship. Just worth considering what you'd do if a,b,c happened so you don't get too disappointed if it doesn't work out. I've got some strumpy females, and although introductions have always worked out, there are often times when everyone has been so upset and frantic that you wonder if you've done the right thing!