The key factors to consider are:
The number of times customers contact a company -- call volume -- and the nature of those contacts can vary tremendously. The other factors noted are a function of your business strategy, operational strategy and contact types. Taking all of this into consideration, the ratio of customers to staff is a "custom" calculation for each company. The best approach is to create a model, make assumptions and review those assumptions with people who have some insights on the products or services you are offering and what types of contacts you will create. You may be able to benchmark with others in your industry, but take that with a grain of salt, as the assumptions and inputs can vary significantly.
This was first published in November 2007