Based on a rectangular room, with multiple seating this is what Todd Welti’s research showed (I assume his research was more geared towards home theatre application). Google Todd Welti if you’re not familiar with him.
"It can be seen that there is less difference between “good” and “bad” seats, and that there is less low frequency support for some configurations compared to when the grid is centered and unweighted. Two and four subwoofers at the wall midpoints are still the best configurations overall. Four subwoofers in the corners does not seem as advantageous as when the grid was centered. There is still little or no advantage to using a large number of subwoofers. LF factor still goes down for higher numbers of subwoofers."
"CONCLUSIONS • How many subwoofers are enough?
Four subwoofers are enough to get the best results of any configuration the best results of any configuration tried. Two subwoofers is very nearly tried. Two subwoofers is very nearly as good and has very good low as good and has very good low frequency support as well."
In other words, you may be wasting your money having more than 2 subs, and certainly wasting money with more than 4. The best outcome is between 2-4 subs, but you need measurements to determine which is best. Is it worth spending money to fix a 2-3dB dip/peak in your room by adding a 3rd or 4th sub?
I think you’re better off starting with 2 subs (especially if your listening environment is single sitting for music), spend $100 on a good mic and some cables. Take some measurement using REW (free software) to see if more subs are required. You could save yourself a couple of thousand dollars.