There's no way you can come up with even a ballpark number of DBA's per server. With the right tools, a well set up infrastructure, reliable networks and well written applications, a single DBA could support a very large number. But if the network is flaky and the apps are dodgy, and there's no investment in monitoring and response tools, even a single server can become a handful.
I think a better question would be to ask what a DBA needs to be able to manage a large number of servers well?
Gotta agree with this one.
Also, the number of servers per DBA will vary depending on what they are used for. A server that is purely overnight ETL and reporting is much easier to manage and maintain (in my experience) than one that's getting hit by thousands of transactions per second of OLTP activity from a dozen applications.
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