U.S. citizens who are living here, even if they are not Canadian citizens, can be affected by the obligation to identify themselves and to be reported on, because the U.S. tax base includes taxation on the basis of citizenship.
In terms of how many U.S. citizens are living in Canada with dual citizenship—that is, Canada-U.S. citizens or otherwise—we don't know the number. It's commonly suggested there are a million, plus or minus.