Bass is about nothing more than displacing air. In general, the more subwoofers you have (with all things equal), the more air you will displace, and the louder you will be.
Obviously 12 12s in an absolutely terrible box on 6,000w wouldnt be as loud as 2 18s on 6,000w in the right box, but for an everyday setup that plays a range of music, I prefer to run a bunch of speakers.
I tried a setup with 4 BTL 18s on like 17,000 watts. Was ok, but it stresses the subs to run that much power. Then I went for a setup with 24 15s on 20,000w and was much happier with the results for a daily driver. It played a wider range of music, was more efficient, was louder, and since the subs barely had any power to them, it would play all day with no equipment stress or failure.