Every SIM number is unique. It's usually 13 digits long and ends with a letter. You can choose to ignore this letter when asked to enter your SIM number.
On your SIM
You'll find your SIM card number on the packet it came in, or on the actual SIM card itself, on the opposite side of the chip.
On your phone
Some phones let you find your SIM number via their settings:
From the home screen, tap Settings
Your SIM card number is under ICCID
From the home screen, choose Settings
Scroll down and tap About or About Device
Choose Status or Phone Identity – this will depend on what model you’re using
Choose IMEI information
Your SIM number will show as either the IMSI number, ICCID or SIM ID