When the SPI is a master, it means it can talk to any device on the bus. You may have 1 device or several. The way the device knows you are talking to it is to enable the chip enable pin on the device, Most devices have you make CE low to enable it.
This is done by any General Purpose IO Pin set up as an output.
To talk to the device you need to makes the CE Low, then start using the SPI commands. All other devices that are not selected are in low power states and ignoring the bus.
When you are done release the CE (make it high) line and it puts the device in low power mode.
Since nobody but the designer knows how many devices are on the SPI bus, the interface does not have dedicated pins for that purpose.
