Hi

Maggiesmile!

Easy!

Look for this demo in raspi https://youtu.be/QmydgunbpPc

Follow the links for undernstand the serial communication.

Atmega328p has just one serial port. You can use other board, like BBB (10 x uart)https://beagleboard.org/

You can use this post to grasp the concept of (de)mux: https://jungletronics.blogspot.com/2016/05/06-arduserie-expanding-arduinos-pin-w.html?q=demux

Hope that helps o/

Have A Good Day! Bye!