IT benchmarks

There is NO true staffing benchmark or standard relating to staffing that will work in all organizations. There may be "averages" across industry, but that is all they are averages. Some organizations need more, and many organizations need less.
Staffing is entirely organization- and environment-dependant and sometimes even industry-dependant.

There is no way to say that you need 1 IT person per XX number of users, with out taking into account such variables as:
skills and experience of the IT people involved;
requirements of the customer base;
industry-specific requirements;
legal compliance issues;