When the Student Visa Compliance team report your course completion to UKVI, they will email you confirming they have done this, and the email will include your CAS number.
This CAS number is the one you originally received from the University of Greenwich just before you made your visa application. Provided you have access to the email account it was sent to, you should have the CAS number. However, if you cannot find it, you will be sent it again after you have successfully completed your course.
For more information, please see our Graduate Route webpage.