In Legion I had two per class at max level, but I've scaled it back to one per, and occasionally level some Allied Race toons up a bit. I have a bunch of 110s that are mostly there for old raids and the occasional time walking.
For my 'active' toons, I run at least one M+ key per week on every toon (though I didn't get them all this week due to X-mas and getting Word Quest event done on every toon for the 1k resources). Most of them don't have much in the way of essences, so I mostly use the M+ essences for main slots.