Whether you should have 1 or 2 subwoofers depends a lot on your crossover point. If the crossover point is 60 Hz and the slope is 12 dB/octave, you will be down 12 dB at 120 Hz... that's about the HIGHEST value I would consider as a crossover point. If you use 80 Hz or less as the crossover point, a single subwoofer will create a mono soundfield for sounds in the 90 Hz to 140 Hz range that will reduce spaciousness in the sound mix (if there is any). It is my experience that I can identify left from right from 90 Hz and up. Most people say bass gets monophonic at 100 Hz and lower even when there are multiple subs, but I find 60 Hz to be a better cutoff for me to maintain all the stereo sound properly. If you really need to use a crossover point of 80 Hz or higher to get the proper blend of subwoofer and regular speakers, I would recommend 2 subwoofers. If you find a 60 Hz or lower crossover point workable, you can do that and use a single subwoofer. How to determine what crossover point to use: Using manufacturer specs and test tones, determine the frequency were your main speakers start their bass roll-off. You want to find the point where the speakers are nominally -3 dB from their full bass output level... so if you measure 75 dB at 100 Hz and 72 dB at 90 Hz, your crossover point should be the -3 dB point, or 90 Hz in this example. You want the subwoofer and main speakers to both reach their -3 dB point at 90 Hz in this example. If you have large-ish L&R speakers that reproduce solid bass down to 40 Hz and they measure -3 dB at 35 Hz, 35 Hz would make the best crossover point. And a crossover frequency that low means you will never know the difference between 1 and 2 subwoofers as long as the 1 subwoofer you do need can produce the SPL you want/need.
Sealed subs are best sounding for music, but they have the LEAST bottom-end extension. You get the least amount of distortion in the bass from a sealed box but you are unlikely to get much output below 20 Hz from a sealed subwoofer unless the box is quite large. Ported subs almost always will play louder and go deeper than sealed box designs of similar cost. But ported subs also have more distortion in bass frequencies. Many ported subs come with 1 or more port plugs that allow you to tune the subwoofer. Hsu Research subs, for example, have 2 ports. You get 2 closed-cell foam port plugs so you can operate the sub with both ports plugged for music, and open 1 or 2 ports for movies. Best of both worlds. Other manufacturers MAY (or not) offer that flexibility too.
How low to audio frequencies go in Movies? The disc of "The Edge of Tomorrow" (Tom Cruise, Emily Blunt) has a sound effect that begins before the Village Roadshow logo at the beginning of the movie fades out to the movie... it starts around 40 Hz and slides down to 10 Hz. Below 14 Hz or so, you hear nothing, but it feels like somebody turned a fan on... the subwoofer moves a HUGE amount of air at 10 Hz with no detectable sound. The subwoofer doing this in my room is a Hsu Research sub, the least expensive model having a 15-inch driver... the VTF-3 Mk5 HP (currently priced at $909 plus $110 shipping in satin black). This sub has 2 ports and 2 port plugs so it can be used as a sealed, 1-port, or 2-port sub just by inserting or removing the port plugs. The only question is whether the volume of your space needs 2 subwoofers or not. You can always try a single sub first to find out if the bass levels you want/need are OK for your space and add a second sub only if it feels like a single sub isn't filling the space adequately. Of course if your main speakers will need a crossover at 80 Hz or higher, and you want to maintain maximum stereo separation into the bass, you would need 2 subwoofers, each fairly close to one of the main speakers (within a couple of feet if possible). With a single subwoofer, if the crossover frequency is 60 Hz or lower (80 Hz in a pinch), you should place the subwoofer between the L&R speakers to prevent pulling the stereo image to the left or right with an off-center sub. A single sub can be farther back or even closer than the main speakers as long as it is equal distance to the L&R speakers. Some people deal with large space volumes by placing the subwoofer behind or beside the main seat. Using this placement, you can get stupendously loud bass from a single subwoofer. With placement close to the listener, you lose all the effects of room modes and you hear the most linear bass possible, much more linear than the bass would be with the subwoofer placed farther out into the room. You do have to use careful/precise level-matching between the subwoofer and main speakers so the bass doesn't sound overpowering at the listening position. In most cases, placing the subwoofer close to the seat or sofa you use will induce bass resonances into your sofa/chair further making the bass sound realistic as powerful bass in real life does the same thing... resonates/vibrates whatever you are sitting on.
