You need 2 pieces of information.
1. The audio codec/compression you will use for the calls. Typically it is either G729 or G723. You can look up on the internet to find out how much bandwidth they each need. Make sure you do not forget to add in the IP overhead like packet header and such to determine complete per call bandwidth required.
2. You need to know how much bandwidth you have available to you at the location. You need to know both up and down bandwidth.
Once you have both of these it is simple math to divide one into the other and that will give you the number of calls.