Don't know exact figures and this article is out of date, the format changed and the grind was less of an issue, so the number of legend players is likely higher now but this still gives you a rough indication.
In a nutshell, not many. Data I've seen seems to indicate between 0.5% and 1% of players reach legend. Even if you assume these numbers are way off, it isn't going to jump to 20%, the vast majority of players don't hit legend and I would imagine even less do so consistently.
Anytime I've seen data around this, it's always quite surprising to me how many people populate the lower ranks. I assume some of it is people who don't play very much, maybe dip in a few times through the month instead of playing consistently and aiming to climb.
But I also think it shows that people go very over the top when they suggest there's no skill involved in the game. There absolutely is, which is why some people just can't get past silver/gold and others cruise to legend within a day or two every single month.
It used to be a lot harder to reach Legend for sure. You could reach Legend right at the end of the month and be in "dumpster" Legend and your rank would be inside the top 2k. Now a late legend won't even make the top 20k.
A big factor was in the past it took 25 wins over losses from the equivalent of D5 to reach Legend compared to only 15 now. Go even further back to when there was no safety net and it was even tougher because every single game you'd face a tryhard netdecker rather than all the gimmicks you see now of people having fun safe in the knowledge that even a terrible streak doesn't drop them lower than D5.