To my knowledge, FBI is valid for 6 months. But then again for some cases if application is just about to be approved and FBI expires, they will not request from you again. If you submited your FBI and State clearance report with your application, and if case is not going anywhere and clearance reports expired or just about to expire you can order it again. Why? Because if immigration requests again you will have it right there on hand and can mail it right away.
You also need to be aware that nowadays it takes up to 12 weeks to get FBI clearance report, so order it early because you do not want to wait.
You do not need to provide FBI with your ID number if you do not have it. All you need to do is tell them that it is for Canadian Immigration Purposes.
All the best!