IT Staffing Models

I work in municipal goverment agency of about 160 people spread over 20 locations within the same city. My agency is re-assessing its IT staffing structure,in response to increased budgetary pressures. At present, our IT Support Functions (Help Desk, IntraNetSite Maintenance, database management, server operations) is completely outsourced.

Are there any “rules of thumb” or other simpified models that a (non-technical) manager for an organization can apply to estimate what its optimal (or perhaps minimal)level of IT support staffing should be? Any suggestions or guidence would be appreciated

Thank you.