Well now my first response makes me look like an ass : P
...on topic, they've never shared that info and I doubt they ever will. Almost every serious player I know (playing more than 3 nights a week, active in at least one "endgame") subscribes.
I would guess about 10%. While most of the players I play with and are within my circle of game friends are ESO+, there are tons of players that are much more casual that I generally don't interact with regularly.
My guess is around 15%. Most I'm guessing think spending an occasional buck or two is more than enough and when/if they buy crowns they buy them on sale.
These same folks spend twice that every week on cigarettes and coffee.
There is nothing noble in being superior to your fellow man, true nobility is being superior to your former self
-Hemingway
Most of the people ive met in game have ESO plus. Id say 30-60% of all players have ESO plus. $15 a month isnt exactly budget breaking for most people who can afford a modern PC and decent internet.