It's not about how many people, it's about AREA. How many square feet/meters?
I tend to agree that for most contemporary genres, there is no such thing as "too much sub" in the eyes of the band, DJ, and a good bit of the audience. One can always turn them down or unplug half of them (-6dB, instantly), but one cannot make more appear from thin air when needed, so there is that.
One of the 'sleeper' subs for almost a decade was the JBL SRX728. Not quite up to the VerTec 4880 sub, but close. Fifteen years ago there were tons of VerTec 4888 and 4889 rigs with SRX728 subs. We just sold our last 16 of those subs a couple weeks ago.
All subs are not created equal, either. There are plenty of 2x 18" subs that didn't perform as well as the 728, and some others that were better (and more expensive). A long term plan is to buy the highest-output subs, because that's the base upon which multiples will be built.
To double the SPL (+3dB) you need to double the cone (piston) area. Doubling the available current (another amplifier, usually) can give another 3dB. It's advantageous to start with better performance if you're going to be needing more subbage in the future. If one needed 4 subwoofers to achieve a specified output level, using subs that have 3dB more acoustic output with the same electrical input would mean one would only need 2 subwoofers. Those more efficient/sensitive subs will cost more, perhaps 3dB$, but then need only half the truck/van/trailer space, half the storage space, etc.
Have fun, good luck.