I am using arduino uno for the first time to control multiple DACs. But i am a bit confused about the SS line which is mentioned pin10 on arduino uno but in case if i have multiple devices should i use other free digital pins like pin 9,8,7,6 etc.All my dacs are SPI compatible.
You can use any pin for SlaveSelect, and each SPI device needs to be controlled by a separate SS pin.
You do not even need to use pin 10 for any of the SS lines, it can be used for other purposes. But pin 10 must be set as OUTPUT to ensure that the UNO operates as SPI master.