I don't think we've ever had a CB with a combine invite. Collie and Watkins were the last speed guys to get invites.
Doesn't mean we don't have any. Michael Davis (starting CB for the Raiders) is a 4.4 guy. Trinnaman has been mentioned. There have been plenty of others with speed but not other things needed to get an invite.
There are plenty of kids out there who can put up 40 numbers but are not good enough overall FB players to get invites. Percentage wise it is low, but in terms of actual numbers most teams have some of those guys.
BYU probably has 2-3 at any given time. Certainly not more than that, and sometimes less.