The system Wilson did his first listening events on had 12 ceiling speakers. Atmos and DTS:X can be coded with a set channel layout (generally 4) or, as is more common, let the sound be object based. In the latter scenario the processor can theoretically support whatever speaker configuration there is. Wherever the object moves in the room the processor will channel the sound to the appropriate speakers.
So, in the case of ceiling speakers, 4 is an ideal minimum since certain mixing is coded specifically for it and these mixes unfortunately won't play on additional ceiling speakers unless the processor is capable of upmixing it. But unlike base channels you don't really have to try to squeeze in more than 4 ceiling channels since there isn't discrete material trying to be folded into other channels like we run into with the base channels. The use case for more than 4 is for rooms large enough to need it.
So, in the case of ceiling speakers, 4 is an ideal minimum since certain mixing is coded specifically for it and these mixes unfortunately won't play on additional ceiling speakers unless the processor is capable of upmixing it. But unlike base channels you don't really have to try to squeeze in more than 4 ceiling channels since there isn't discrete material trying to be folded into other channels like we run into with the base channels. The use case for more than 4 is for rooms large enough to need it.