Because I needed a small subwoofer that would fit under my modified-buffet stereo cabinet, I chose to start with one sub and was immediately pleased with the enhanced SQ.
However, after listening for a few weeks I decided something was off, the bass subtly imaging more to the side with the sub when listening at louder levels.
Since I have adequate crawl space height, I opted to hardwire a second, identical sub on the other side of the back of my room.
The overall SQ is now more balanced and richer than it’s ever been.
If you have doubts and/or budget constraints, plan for two but start with one and draw your own conclusions.