Would you choose 1 ou 2 subwoofers for a stereo set?

Agree with @jasonbourne71 and @mijostyn - two bigger subs. Going from one to two subs made a significant difference in both the sound of my system and my level of enjoyment from what I hear. You need big and fast subs that goes low, to augment the bass frequencies provided by your main speakers. Maybe poke around and find out what other 802 owners are using successfully.

Two if you can. Much easier to tune.  I actually like 12 inch or smaller for better control.   I use an Antimode Cinema for DSP control.  Works very well and is only in the sub path. 

Rel, Rhythm, and SVS appear to be the leading subs these days. Personally, I like low Q sealed subs. I think they blend better than ported and have less group delay.  Even my HT, my twin 10 in push-pull sealed can make special effects move your stomach!  

"Fast" is a bit if a bogus term used to describe other parameters.   They have their frequency response and their Q. Both measurable.  The sub, like ANY speaker just needs to follow the signal. No delay, with as little distortion, and as little overshoot as possible.  A lot of sub "fast" is not crossing over your main speakers and sending too strong and too low of a signal to them. Boo-Kou distortion. Not having a subsonic filter on a ported sub can make a mush out of it too. 

The magic of a sub in a Stereo is not just deeper, but cleaner.  802's go plenty deep. Surprised you need a sub at all.  Not much music below 40 Hz. 

Room ambience below 40Hz on some recordings, and it is nice to experience even with no bass in the track.

Totally agree with two subs over one. If you can room correct the bass you'll be smiling when you are done. Agree with most all of the above comments. good luck

Always two. They will extend the soundstage and give much greater presence.

If you are optimizing for a single sweet spot for 2 channel listening and have a lot of freedom to move the subwoofer, speakers and listener’s position around, you can make it work well with a single sub even in a very large room.

If you don’t have full flexibility of placement mentioned above, restrictions on variable phase, etc. you could add another sub to help the compromised first sub (be tactical about it though without making anything worse).