2 - 4 sub is needed to get a smooth response in a room. More than 4 subs has no real benefit for HT in most cases. As was stated earlier, there is some benefit to a sub being elevated off the floor. If done right, bass traps can be made to go under the shelving to help clean up the sound if needed.
Klipsch RF 7 based HT 7.4, Pioneer SC 35, Acurus 200 Five, Dayton 18 Ultimxa Dual Sub Cab(2), Dayton 18 Ultimax Large Vented Sub Cab (2), on Berhinger I Nuke DPS amps, Aura Bass shakers, Samsung BDP F 7500, Asus/My Book Live HPC 4 TB
Yaquin VK 2100 amp, McIntosh XR 5 speakers, Samsung BDP F 7500