"Today, Scott Guthrie, Microsoft's executive vice president of the cloud and enterprise group, said in an interview, "it's about half now, but it varies on the day because a lot of these workloads are elastic, but sometimes slightly over half of Azure VMs are Linux." Microsoft later clarified, "about half Azure VMs are Linux.""

https://www.wired.com/2015/09/microsoft-using-linux-run-clou...

https://azure.microsoft.com/en-us/blog/microsoft-showcases-t...

However, I could have swore I read semi-recently that Azure itself was primarily running on Linux, but I could be wrong here in some regards.