Unsurprisingly, usage numbers for distros are hard to get due to lack of telemetry and what not.
However, some measurements do exist; like data from ProtonDB. These are used by Boiling Steam for their excellent reports in which some representation regarding usage across distros can be found. Their most recent report can be found here.
Note, however, that the following, as has been excellently touched upon by Boiling Steam, applies:
COMMON MISCONCEPTIONS
Since we hear some of the following comments EVERY SINGLE TIME, let’s address them here and now:
“Duh, it’s not representative of Linux usage in general!”: And nowhere does it claim to be. As often as possible we make it clear this is Linux usage in a gaming context. The usage of Debian and Ubuntu on servers is safe for now, no need to panic.