You are likely to get your BJCP number at the same time you receive your exam results, but you may receive the number before then. The timing varies based on when your data is added to our database. This usually occurs when the exam results are processed, but it can sometimes occur before then in certain circumstances.
Note that your BJCP ID is different than the certificate number you received when you took the online exam. The certificate number is not a BJCP ID; it simply allows you to register for a judging exam.