The number of slaves in a system is theoretically infinite. The limiting factor are the output drive buffers, for the PSoC . This is 10mA source and 25mA sink, and the capacitive or/and current requirements of the input pins, which can only be determined by test. What it boils down to is as you add more load on your output pin, eventually, you'll miss a timing requirement somewhere. Nevertheless, you should easily be able to drive devices in multiples of ten.