5.1, 7.1, Atmos: what the numbers mean
The first number is speakers around you, the second is subwoofers, and the third, if there is one, is speakers overhead. So 5.1 is five speakers and a sub. 7.2.4 is seven around you, two subs, and four in the ceiling for Dolby Atmos.
More isn't always better. A well-placed 5.1 in a family room beats a badly placed 7.1.4 every time. I'll tell you what your room can actually use.