I have four speakers in my living room and I had spaced those speakers at equal distance from each other by calculating my room's total area. And I consider my speakers are best in quality when compared with my friends ones.
I think it all depends on the size of the room where your AV equipment is in place. Also, the question of loudness is and will always be subjective so to best answer your question, IT DEPENDS on a ton of factors. Just be satisfied with what you come up with.
There are all types of entertainment centers. Are you asking about a Home Theater entertainment center? Stereo Hi-Fi entertainment center? Video game entertainment center?
Some Home Theaters have a 7.1 setup, 5.1 setup, 3.1 setup, and even a 1.1 setup. The first digital is the quantity of surround speakers and the second digital is the subwoofer. I don't think there's any minimum amount of speakers to consider something an entertainment center, it's more based on what you (the audio and video you see and hear) consider it.