On the Slave Select (SS) or Chip Select over SPI I rather prefer using 2, 3 or 4 GIOP lines to handle a 2 to 4, 3 to 8 or a 4 to 16 lines priority encoder to perform SS / CS on the slaves.
If all slaves have tri-state MOSI/MISO/SCLK lines, there is no need for an additional tri-state bus driver to isolate these lines on the slaves. With this configuration the master can handle for instance 16 slaves with 4 extra GIOP lines.
The number of slaves plugged into the MOSI/MISO/SCLK is restricted only the cumulative capacitance on the bus that increments tau and dampers the signal. Also take into account the propagation delays introduced by extra ICs to comply with SPI CS/SCLK and maybe slave IRQs on the master.