New sim cards do they match the number on account before activation

Hi, I have ordered 2 sim cards from seperate 'new' accounts for 2 different phones. I'm now worried that I won't be able to work out what sim card is for what account. Is the phone number that's appearing on the account assigned to the sim card before activation? Or once I've inputted the serial number and the sim is activated it then gets assigned the phone number that's currently showing on the account? I don't want to break the sim cards up into certain sizes and it not be correct as both phones take different sizes. Thankyou