He said 15 VM’s running for clients. Now you would want to secure these clients from each other, restrict east to west movement. Adding them all on the same domain introduces security risk, reducing them risk and hiding clients from one and other in the same domain would take lots of effort. So just don’t put yourself in that situation and use multiple domains one domain for each client.
Then you start getting things like Azure AD Sync etc. It’s best practice one domain per client. Not trying to make one domain work for multiple different clients.