Well Stated--No Way to Know without Defined Services & SLAs
There is no formula to size an IT shop because they are all different.
What are your uptime requirements? Do you have bandwidth-intensive applications (imaging, multimedia, etc.)? When new staff come onboard, do you pre-configure their machines or take them out of the box?
What back-office support functions (backup, server configuration, etc.) need to be performed and how much time do they require?
What are your actual/historical call volumes to the helpdesk? How quickly are responses required when calls come in? What hours does the helpdesk need to be staffed?
Who will evaluate new services prior to implementation? How will they be implemented (in-house or outsourced)? (This is more of a budget issue than a staffing issue, but you can be sure it will impact IT staff time.)
Once you've done this kind of analysis, you can evaluate the skill requirements and the time needed for each skill.
While this may seem time consuming, it's a task you can do easily in a spreadsheet and it will provide the justification you need to sell your executives on the level of staffing needed.
