TheyCallMeTojo Flarum doesn't collect those statistics directly.
However because we are using Packagist for the code distribution, you can see the download stats there.
https://packagist.org/packages/flarum/flarum/stats gives you an idea how many new Flarum installs are being made (this is the skeleton application)
https://packagist.org/packages/flarum/core/stats (core codebase) will include both people installing and updating.
The stats for each extensions can also give a good idea how active the community is.
There's sadly no definitive stats about "active" installs. FreeFlarum, Flagrow.io and the Flarum lab are third-party services that collect data and give us some partial estimates. https://builtwithflarum.com/ showcases some of the biggest forums. (and I should update it, it still shows "beta 8" for either 8/9/10)