There are certainly no stats I would have access to. You’d have to contact every Munki admin in the world and ask them.
I don’t even have stats on how many Munki deployments are out there — literally anyone can download the software and setup their own Munki implementation without needing to notify me (or anyone else).
Additionally, I’m sure there are organizations that would not want to share information like that. I know Google itself used Munki until fairly recently, and there are other Very Big Companies that use it but may not want others to know their scale.
And remember that Munki is the software deployment “engine” inside several commercial management products, like VMware Workspace ONE, SimpleMDM, MobileIron, and more.
Sorry we cannot be of more help!
-Greg