Can I use 12/3 to run half switch outlet. I have 12/2 run to my bedroom from my panel(arc-fault breaker). Is 12/3 will make arc-fault breaker trip. Or I have to run 2 12/2 from switch

You are not making sense.

Whether the cable is 12/3 w ground or 12/2 with ground the maximum amperage for the cable will be 20 amps.

When you say you want to use the 12/3 with a half/receptacle are you talking about a switch controlled receptacle on the bottom and an always hot receptacle on the top?

Are you trying to set up a multiwire branch circuit (which would actually require two circuit breakers)?

We need more information on what you are trying to do.

I already figure it out myself.
Thank you for your help.