I have long been an advocate of stereo subwoofers: one for the left and one for the right.
Today, I am a bit more cautious when making such a recommendation. Why? Because while you can never go wrong with two subwoofers, you might not always have the budget for two great subwoofers—and thus, you have to choose: two ok subs or one heavy beast?
How to decide?
My rule of thumb is based on frequency response. How high is the subwoofer being asked to go?
Back in the day, most subwoofers I worked with started rolling off at 40Hz. When your low pass cutoff point is that high, subwoofers become directional—requiring two subs for a convincing soundstage.
But today, using a speaker like the aspen FR30 or FR20 there's no need to ever go above 30Hz. In fact, with either of those speakers there's no need for a subwoofer at all
...if it weren't for the room.
It is the rare room that can support smooth frequency response down to 20Hz from the main speaker pair. There's going to be big dips and that's where our friend the subwoofer can help.
If you don't go much above 30Hz, it's effective to get away with a single well-placed subwoofer to fill in those dips.
Is one enough?
Depends on your main speakers.