Obviously there's no way for us to know for sure, but there have been several articles published about VMP that used numbers provided by VMP that the writers thought were credible. It's the one thing about VMP that I don't really see any reason to doubt.
I think it was the Billboard article (now behind a paywall) that most recently provided rough number estimates in the ~30k range, with mentions that the Classics and R&HH tracks were each selling in the ~5k ballpark. But prior to that, they were floating ~15k numbers in 2016, and even have publicly discussed a mistake in 2015 when they overestimated how many copies of an AOTM they were going to sell when they ordered 25,000 copies.