My answer to What is the maximum number of slaves that can be connected to the Master in SPI (serial communication …
Answer by Shaunak Agastya Vyas:
The number of SPI slaves is not limited. But at the same time, it is restricted according to the number of hardware pins of controller. In fact, you can add digital muxes (multiplexors) and control more slave devices than you have digital pins on the controller.
However, the SPI was design for communication over short distances within a box. So, the physical size of the bus can become a problem (bus capacitance, EMI).