I don’t think there’s anywhere near accurate, at least for Verizon. Just in DC and Baltimore, there are Verizon mmWaves sites all over the place. That being said, I don’t think mmWave is critical for most users with a cell phone, but Verizon deploys it pretty widely in major cities. I think a more relevant metric would be the percent of people who encounter mmWave coverage daily - since the places that it is have much higher density, I’d guess it’s something like 30% of the US population.
I know other carriers deploy it significantly less.