I recall a few years back when a respected engineer concluded that the "optimum" number of subwoofers in a space is .... fourteen!!
That’s only about a dozen more than most mortals can cram into a space. Not to mention budget and "wife acceptance factor." But, boy, just about every square inch of the room would be "the sweet spot" -- as far as the low end is concerned.
Back in the days of serious car audio competition (yes, SQ was a factor in the EARLY days) the folks at Rockford Fosgate came up with what they referred to as "transfer function." They sent plans on how to build a small tower bass enclosure that would sit in the driver’s seat with the woofer located at the approximate driver’s (the one that aims the vehicle, not the raw speaker) ear level. Then, induce a bass signal into the box and physically move a mic around the vehicle. The loudest measurement was the best place to put the woofer. Often, we had to choose "Plan B" due to the impossible logistics of building an enclosure in the ideal location. (Like where the passenger likes to place her Kohl’s shopping bag). It’s kinda of the "crawl around the room and see what happens" process -- in reverse. Those with a little time on their hands might want to try this, just for fun.
Or, use room analysis software, which would be faster, more accurate, but less fun. Nobody invites friends over for "room correction" parties. As an alternative, having 7 of your closest friends moving about on hands and knees to see who wins the "I found the hot spot!" competition would be far more entertaining.