I try to find a benchmark for the number of calls per month for a servicedesk for 6000 PCs. SD handles all types of incidents regarding the PCs ( Hardware, Office Software) + related core infrastructure ( WAN, Email, etc)
Who can help?

I don't recall ever happening upon such a benchmark, but then again I've never specifically looked for one. I think the number would vary greatly depending on the industry, the robustness of the existing infrastucture, how well the PCs were selected and setup for their targeted use, the degree of end-user self-service features (file restores, password resets, etc), and the level of end-user training.

Have you inquired with any of the following resources?

Service desk software vendor.
System integrator specializing in provisioning service desks.
NetworkComputing.com magazine.
TechRepublic.com
Help Desk Institute / Support World magazine