Do I need to transfer my SIM card

You don’t NEED to, but, unless your new phone is an iPhone 12, transferring the SIM is the fastest and cheapest way to activate the new phone. Once the new phone is activated, with or without the old SIM, the old phone will stop working. If you are getting an iPhone 12 get a new SIM or activate the phone’s eSIM with your carrier.

This link might be helpful → Transfer a SIM from your previous iPhone to eSIM on your new iPhone - Apple Support