Potential subwoofer purchase-a few Q’s

Rythmik makes good subs.  The lowest I've gone on their food chain is the F15, but I would expect the entry level models to sound and operate similarly, just with less output.  The guys at Rythmik are very approachable.  I'd reach out to them and get their advice.  I like Rythmik a lot, but you might also consider SVS and Power Sound Audio.  

I am an advocate for multiple subs, and if you go that route, the entry level models will work great.  If you're only going to get one or two, you probably want to save up and get something with more output for that space.

It doesn't matter if you rent or own your home.  If you "own" a condo, putting a bunch of subs in it may not be the best idea.  If you rent a house and it's well insulated, not 3 feet away from your next door neighbor's house, and you don't play everything at top volume at all hours of the night, you should be fine.  Exercise common sense and don't get put off by some of the condescending posts.

how much better is 4 subs than 2?

Much better.  Seriously though, I'm not sure how to quantify it.  Two subs can be great for movies if you just want to have that visceral feel of the bass effects in the movie's soundtrack, but for music having four subs is a subtle and at the same time dramatic difference.  Subtle in the sense that they don't draw attention to themselves, dramatic in that the bass fills the room (not just one or two sweet spots), and is immersive.  I'm sure some of our more eloquent members can describe it better, but I'll summarize with this - if you have room and budget, do it.

Rythmik, IMHO makes, OK subs. Find one of the Martin Logan, "Descent", model subs and listen to it. It will actually change the way you think about subs to begin with.  I tried them all. And the Descent subs are soo far past everyone else it is ridiculous. Maybe because they use "Real" servo correction and not "Feedback loop calculations"? That is just the cheap way to attempt accuracy. Better than nothing but not so great when compared to the real thing. The 7500Watt amp doesn't hurt either. I use three Descents in my main system.

Millercarbon, now you know I use 4 subwoofers but with a smaller system two subs can work fine but two subs is the minimum. I used a two sub system for years until I graduated to line source speakers then two was not enough to match the power projection of the speakers. I did not know the acoustical benefit of multiple subs in terms of minimizing standing waves.