Two subwoofers are appropriate, to augment your main speakers. Appropriate placement takes real work to avoid peaks and troughs. Having said that.
What are you trying to accomplish? Realistic sound? If so two is plenty. When I had two subwoofers my cross over frequency was about 40hz and the volume level less than 20%. If they are not carefully feathered in they create a huge bump in energy where there shouldn’t be one and pinch the sound stage and completely screw up your system.
If you are trying to create an enormous hiphop boom, then lots of subwoofers are great… but if you are working to create high fidelity subwoofers can get out of control really easily.
I used to have two B&W 800 series 12” subs. I had a lot of trouble getting them tuned in, mostly because of me… I kept wanted to augment the bass, and hear it. It was so tempting to turn them too high. I got rid of the went I got my current speakers. My speakers are flat tp 28hz. If I still had my subwoofers… I would have had to have them turned down to the very lowest cross over and probably the lowest possible power.
Crossover frequency is not at all a specific frequency… they have a graduat attenuation… and speakers fall off gently. This is why REL subwoofers are so popular, the supply the means to feather the subs to the main speakers.
More than once I have gone to someone’s house and after listening for a short time… went over and dialed back there subwoofer by a huge amount… and the owner was shocked at how much better their system sounded.