I don't know past numbers but there are a few reasons why an applicant might decline their offer. E.g. being offered the Vanier or other scholarships of higher monetary value that can't be held concurrently with the CGS-D, applicants choosing to go to another university, applicants who were not registered in a degree program at the time of submission and then didn't get accepted to a graduate program (probably rare).
Since there are only a few Vanier spots and results are already out, I would expect those who won both the Vanier and CGS-D to decline their CGS-D offer pretty quickly. In past threads, a few people mentioned being waitlisted and then getting an offer so don't lose hope!!