What is the maximum slaves supported by SPI and how it is limited?
We have two type of SPI,


direct connection
Daisy chain connection


For the first, each device need its own SS pin/ sometimes Chip select(limitation)


link:http://www.circuitbasics.com/basics-of-the-spi-communication-protocol/

For the Daisy chain, The limitation is the delay. Not all SPI devices support Daisy chain, some of them should be configured for Daisy chain operation, for example, some DAC chips.

If you want to send a 32bit command to chip number n, you need n *32 clocks on SPI clock to send data to chip number n.