Geo: Report number of sites on /admin

What does this MR do and why?

Geo: Report number of sites on /admin

...rather than always "Primary site", which is not useful and doesn't make sense since secondary sites proxy most requests to the primary.

Somewhat related to #339262 in that it is a similar root cause: The primary and secondary used to be able to show different things and that made sense before, but now it doesn't.

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