Normally you will order a meter with one set of Current Transformers. A set of CTs is one, two, or three CTs depending on the type of meter. If you already have extra CTs, choose 0 to order a meter without CTs. If you can not get all the wires you want through a CT, either because they will not fit or because they are in adjacent panels, you can order an extra set of CT's. Just be careful that the total current of all the CTs on one phase do not exceed the maximum current rating for the meter.