the Public Mobile sim card has a number assigned to it already. You can't select the sim number. When you activate your account you'll be able to select your new cell number from the list provided at the time you activate.
The number on the SIM card is just a unique ID, it doesn't matter. If you want to pick a phone number, I believe you can select from a set when you sign up. Find SIMs locations here, or order online or from Amazon https://www.publicmobile.ca/en/pick-up-SIM
