This is a common question that is well researched. Let's break down when you can get by with a single subwoofer, and why you most likely want two.
If you are only worried about low-frequency sound at one seat, you are most likely fine with one subwoofer. With the help of ideal placement and EQ calibration, you can maximize fidelity at your main listening position. Just be aware that even with one-seat, room modes can have a big impact on the overall frequency response.
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Most people care about the quality of bass at multiple seats and will be much better off with two ideally placed subwoofers. Post calibration, the two subwoofers act like a team and dramatically reduce unwanted room modes which muddy sound across the entire frequency spectrum. Your low-frequency sweet spot will be much larger and even at more seats. The bass will also seem like it is coming from everywhere as opposed to one general area.
The above guidelines are based on pure physics and apply to any usage, be it 2-channel stereo, home theater, in a dedicated room, or in a practical living room. Of course, the final decision is up to you, but once you go dual subs, you won't look back!