You can get the total server calls with the metric Occurrences : it's the sum of the page views AND the custom links AND the exit links instances AND the download links instances.
be careful! as far as i know this works only for props not evars! if you want to know the server calls where an evar is set, you should take "[evarxy] instances"
this is due to the reason that most evars persist longer than 1 hit and therefore you have an "occurrency" on the next hit even the evar is not set ...
A server call is basically a request sent to Adobe Analytics, it goes from the default pageview, custom link (download, exit, custom), video tracking call (which is basically a custom link customised with video calls) etc.
A pageview server call is generated when you call s.t() where a custom link tracking is generated when you call s.tl().
You might as well call other functions but ultimately they will make an underlying call to s.tl() at some point (per the content of the requests that I have seen). So each time you call a functions to send an analytics request this equals to a server call.
It is important to differentiate primary server calls and secondary server calls. When you do an Adobe Analytics implementation you can specify more than one report suite ID in s.account, which is called multi suite tagging.
The first report suite ID in the list will generate a request to Adobe Analytics (by default image request for web implementation). This request will count as primary server call and will be billed fully.
Any subsequent report suite listed will also each generate a request to Adobe Analytics. Each of these request will count as secondary server calls. Secondary Server calls are billed with a discount.