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.