If your “new switch” means CO, DXS/DCS/DAX, cellular, ATM, key system, or hybrid, ignore the rest of this message. Assuming your “new switch” means PBX, the first decision point is on whether you want it to be non-blocking (you should). From there, you must estimate the amount of traffic that the unit will process at any one time, and estimate how much of that will be intra-switch vs. external traffic (Note: If this PBX will perform any tandem function, you’ll need to account for that, too–easy to do; just double the traffic volume for all tandemed volume). Next, contact various PBX vendors.
Get each PBX vendor to tell you how many units of traffic can be carried in each time slot of their PBX’s center stage switch, and how many total time slots are in each switching module. Calculate the total number of timeslots you need, divide that by time slots per module, and you will have the approximate number of carriers you will need. Determine how many extensions you will have, how many outside trunk lines or tie lines you will need, and then whether you can fit that many circuit cards and trunk cards into that many carriers.
If so, order and provision away!