In theory, dozens... since the master PIC can sink/source 25mA and the slaves have relatively high impedance (2K-10K) inputs. However in practice this will depend on the Data Transfer Speed and the total loaded circuit capacitance. You 'may' need some Pull-Up's on the Bus. If you're breadboarding, you'll discover if it's needed, if you're laying down a PCB board as a starter, there's no harm in adding the holes for a SIL Resistor pack in case it's needed. I can confirm FOUR PICS on a (small two-wire) Bus without problems. Watch you don't use a Port on the Master which has an Open-Drain pin - otherwise you WILL need that external Resistor.