2x 10 or 1x 15... remember condo
Just makes me more think should you have a sub or sound system at all
Concrete/soundproofed construction I hope? I haven't lived with shared walls in quite a long time, altho when I lived in SF the next house was literally a few inches away on the sides....but that was before I had real subs, too
If you're going to use a sub, really doesn't matter the number particularly, it's the general levels involved in using them
Smoothing with multiple subs is a definite advantage, not so much an increase in spl thing (unless you want it to be louder of course). More depends overall as to which sub as to your spl and extension goals. Two of the 10" Monolith THX subs is quite good to start, and not really a lot to be gained from there in a general sense.