In responding to a question about planning your IT staff requirements in LinkedIn Answers today, I realized two things:
1. This could make a good blog post.
2. I love talking about the mechanics of IT.
One of my favorite answers to this quandary is Mark Verber's "How Many Administrators Are Enough?" article from a 1991 edition of Unix Review magazine. It's a great discussion of how to plan and control your IT staffing needs, but it's mostly about principles. And the 24x7-site staffing model, which concludes that you need fourteen people may cause unneeded consternation if you're starting with an IT staff of, for instance, two.
One of the great challenges in IT management is keeping IT and non-IT folks in sync, and my impression is that most IT managers resort to very simple formulae, like "1 helpdesk staff per 50 employees". I've found that this kind of formula breaks down very quickly as organizations and technologies change, so I've tried to use something slightly more involved.