I don't know of any published numbers about specific user count (would be very interested to see them), but in terms of deployments it's gone from more than half on prem in 2018 to ~16% in 2023. Of that 16%, I wonder how many users are both willing and able to use an alternate client?

Of the organizations that have Exchange Server on premises, I'd bet the lions share are hybrid, with regular user mailboxes in the cloud, using the server(s) for application relays, etc.