The demographic and power levels I am using are for D&D/Pathfinder levels, but the overall demographics don’t change, and this gives an idea of how many and how powerful wizards are overall in the world.
The number calculations were achieved by basing it on current real world higher academic tiers. 1-2 would be those going into college/university. 3-6 would be those with a Bachelor’s. 6-10 would be those with a Masters. 10-20 would be those with at least one Doctorate. The math was calculated using general pop statistics (levels 3-6), slightly modified Chi-Squared distribution (6-10), and straight Chi-Squared distribution(10-20). I do think this is a much closer approximation to what an actual global academia of Wizards and their power would be like population wise.
As a minor aside, the population of the United States in 2016 was 323.4 million, which I think lends itself nicely to do some comparisons. Namely, according to the 2016 Bureau of Labor Statistics, all Protective Service Occupations (law enforcement, firefighters, lifeguards, animal control, security guards, etc.) only make up about 1.1% of the population of the United States. So if you were wondering how common a Wizard would actually be, that’d be the closest comparison.