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.