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?