Model for 'Budgeting' Staff to Provide the Best Service

A service itself cannot be inventoried since you can't have a warehouse full of haircuts or massages. So, the service itself depends on staff to meet demand - and this leads to a decision on how to "staff up". This depends on the skills and capabilities of the staff.

You could plan to have two of your best people or three of your average people, or any combination thereof.

So, what kind of calculation or model would you use to ensure that you have an appropriate "inventory" of staff available for a "probable" case of demand?