There are numerous factors to take into account. As a "general" rule do not install more than 7 DS1 circuits into a port network or media gateway. Quick Reason why - 7 DS1/ISDN-PRI = 161 channels (FAS) or 166 (NFAS with 2 Sig) - Each PN/GW = 512 timeslots or 484 usable after subtracting overhead which = 242 simultaneous 2 way conversations. Depending on call recording, announcements, analog, inter-PN/GW traffic etc. you don't really have much overhead left. You can go beyond 7 but you really need to know exactly what you are doing with no variance.
So assuming you have a maximum of 7DS1 circuits per PN/MG you can take up to 161/166 simultaneous calls. Traffic however isn't as simple as the number of channels. All may not be available (such as dedicated outbound) or programmed in the service provider network for use. You need to have a complete understanding of all these values to determine what you have the capability to process (don't forget potential mandatory requirements such as recording channels.
This basically gives you the channel capacity but not necessarily the call handling capabilities. Agent count, wait time, average abandon rate, call blockage, etc. will all factor into what you can actually process.
This is not traffic analysis which is determining actual traffic into your system. This is simply channel capacity. To plan capacity you need to understand factors such as the expected grade of service (GOS), and average call duration. There are several tools available freely out on the internet for traffic planning. For details on the underlying calculations get ahold of Basic Traffic Analysis by Roberta R. Martine.
Of course let's add SIP into the mix. What I have seen lately is many carriers have insufficient capacity to fully support what they are selling. Regional and National performance issues are normal with some carrier's SIP service. Capacity testing becomes more critical then capacity planning. If you test a 10 meg circuit for 50% capacity with voice don't assume you can use 70% capacity for voice. Router choke points are not uncommon. For example I have notices greater then 50% reduction in throughput of rated capacity on some routers once adding encryption and passing voice (small packets).
