Every SIM has a unique serial number which is used to identify it. You’ll need to know your SIM’s serial number to perform a SIM swap and transfer your number to a different SIM card on the same network.
To transfer numbers between SIMs on different network, you’ll instead need a PAC (Porting Authorisation Code).
You can find the serial number printed directly onto the SIM itself. The number will be between 12 and 20 digits long, so it may be printed over several lines even though it is all one number.
If the serial number isn’t printed on your SIM card, you will be able to find it on the packaging or in the documentation of your SIM.
If you are unable to find your SIM serial number, you may be able to get it from your network by contacting customer support and giving the phone number associated with your SIM.