Welcome to the forum ! SFF virtualisation servers are definitely possible, although depends a lot on what you want to achieve.
Just some VM hosts ? It goes all the way from a pair of SBCs to rack servers.
One do-it-all machine ? From a NUC to a rack server is possible.
I'm currently using a Silverstone six 2.5" bay case (that has 10 disks/SSDs) but am migrating to a Dell R630 soon.
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Hope to see you around, enjoy the wonderful world of SFF !