A couple other things to consider concerning the number of subs needed are how many people will be listening and where the seating positions are in relation to each other. One reason for multiple subs is to smooth out the bass response across different positions in the room. So the more seating positions you have and the more spread out they are, the more subs you need to get good bass response at all positions.
With X-Statiks, if you only have one seating position, like in my man cave, one sub is all you need. You can put the sub in the listening position and do a sub crawl which is literally getting down on your hands and knees and crawling around the room to find where the bass sounds best. If this is a location that will work for you, put the sub here. If it won't work there (out in the middle of the room or something), keep crawling around until you find a spot with good bass response that you can live with.
My living room is similar in that there are only two seats and they are next to each other. One sealed sub works well enough for us here also. Initially I had the sub on the front wall but found it worked better behind the couch so that's where it's at. For my wife and I, this is fine for both music and movies. The sub is as 12" sealed GR/Rythmik servo sub with an A370PEQ3 amp. The sub has solid output down to 10Hz which vibrates the couch like there was a ButtKicker attached to it. The low end output is plenty for us when watching Star Wars or Marvel movies. However, it is not enough to rattle the pictures on the wall or the windows at the other end of the 20' x 25' great room. If more output is desired, another sub positioned where it augmented the current sub could be added.
Another reason for using two subs would be to connect them in stereo, one for the right channel and one for the left. This improves the stereo experience but positioning is more complicated. With this type of setup you might want a third sub dedicated to the LFE output for movies and positioned where it works best for that purpose. This is how my man cave system is setup. I have stereo OB subs up front and a sealed sub in the back for LFE duty. Currently the rear sub only plays when watching movies.