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

2 - 12” and my vote goes to REL. For music, REL are so much better and easy to blend with your main speakers than JL’s. 

Well, I have (4) JL-Audio F-113 subs near the corners in my room. I like being able to set the calibration adjustments for each in the room independently.

ozzy

I had one vented REL and was happy vs no sub. Moved to two 12" sealed Rythmiks and was much happier. Now I have 4x 12" sealed Rythmiks and the improvement has been great again!

If you have total flexibility for positioning subs then 2 might be enough. I don't,  as the system is in my living room, and 4 subs is much better in my case. I also implemented MultiSub Optimizer software and a MiniDSP 4x10HD. When I had 2 subs I simulated in MSO using 2, 3, 4 by moving the 2 subs and taking measurements. 3 was better than 2 and 4 was better than 3. Went with 4 and haven't looked back. End of the subs path for me.

I suggest DSP-based active crossover, time alignment and room correction hardware and software such as Danville Signal dspNexux, miniDSP or DEQX

• Time Alignment in 5uS increments
• Sample Rate Conversation and Jitter Attenuation
• Room Correction and Bass Management
• Active Crossovers
• low pass the subwoofer and high pass the full-range loudspeaker 

I use two 15” sealed Rythmik subs in a very difficult room with great results attributable to DEQX. Use a professional (Larry Owens) to calibrate the DEQX and plan on spending three hours of your time. 

Do some research into distributed bass arrays. There are numerous discussions on this forum, plus plenty of supporting articles online.

The basic idea is to use 4 or more subs placed asymmetrically around the room to flood the room with the low frequency information. 

The results are amazing.