Unfortunately, there is no magic formula to do what you are looking for. The number of calls generated by customers depends on the type of business you are in, the type of issues customers have, the type of services you provide etc.
Moreover, as I assume that you want to understand the call volume in order to estimate staffing, you also need to know the duration of each call and how calls are distributed along your working hours. Again, there are no magic numbers or standards that can help you.
You will need to measure the call volume and use it as a baseline for process improvement and growth planning. If you cannot do that (e.g. if this is a planned call enter), you will have to create a baseline using some assumptions about the needs of your customers. Without knowing anything about the type of services you provide and the audience you support there is not much else that can be said.
Once you have initial data, you can use WFM tools to estimate staffing and service level performance, and what-if analysis to assess the impact of changes in call volume. You can refer to http://www.DiagnosticStrategies.com/traffic_modeling.htm and http://www.DiagnosticStrategies.com/EasyErlang.htm for additional information.
If you provide more background information about your call center I’d be happy to offer more specific response concerning estimating call load.