which sub to get

You may only need one sub but it depends. It is a practical compromise though. Two are really needed for optimal results in all regards from a sweet spot. That's why any full range stereo speaker has woofers. 4 even maybe if you want the right bass throughout the room, not just the sweet spot.

A benefit of using even one powered sub is the potential to offload a lot of
heavy work from the main amps and speakers so that alone will often result
in better sound even if there is still likely room for improvement by adding
subs. In many cases the added benefit Of more subs may be marginal
enough to not matter to some. I've run with a single sub crossed over
above 50hz in the past and been quite satisfied. Although bass levels
varied at various room locations. Also I know technically imaging and
soundstage would be even better with two well placed subs but practically it
sounded spot on as needed in the sweet spot.