Are there any numbers on the size of the (Qubes)-Whonix user base? Actually I’m mostly interested in the shares of Windows/non-qubes-linux/qubes or ova vs qcow2 file downloads. The ova files work on linux too so this will be hard to separate, but you have to start somewhere …
Well Qubes alone is pushing 25K:
Most people have set up Whonix templates as part of installation, even though the number doing downloads over Tor is a very small subset. So that gives you a rough idea for that subset.
No idea re: plain Whonix. As far as I’m aware Whonix itself doesn’t keep any stats on users e.g. upload data in the background. So maybe distrowatch could give you a rough idea.