Hello Mina
Before answering your question, I suggest you go through the following two lessons that will most likely clarify some of your questions about certificates:
For this particular question:
It really depends upon the method of authentication that you are using. In the wireless authentication lesson, you will see that for PEAP, you don’t need a certificate in the user devices, while EAP-TLS does.
I hope this has been helpful!
Laz