I'M BUILDING two eight channel christmas light controllers with single relays.so my question is how do i power all eight single relays with one arduino.

Seems you need a relay module for your arduino

https://randomnerdtutorials.com/guide-for-relay-module-with-arduino/

https://i2.wp.com/randomnerdtutorials.com/wp-content/uploads/2016/12/relay-module-pinout.jpg?resize=712%2C310&ssl=1