Q: How to find sim card phone number

The contacts are not on the SIM card. They are on the old phone or on backups, iCloud or iTunes. If you are going from one iPhone to another one, you can backup your old phone and then restore the backup to the new one.

If you are moving from Android...

Move from Android to iPhone, iPad, or iPod touch - Apple Support

If the SIM came from an older phone, not an iPhone, then after putting the SIM into the phone, go to Settings>Mail, Contacts, Calendars; then scroll down below the contact area and you should see the link to Import SIM contacts. It doesn't work real good because of the poor way that old phones save contacts to the SIM. Good luck.