I'm going to guess either very few or none, as most people won't notice/care about this issue, and HTC isn't making any money off supplying updates.
Imagine if Microsoft forced Dell, HP, etc. to handle software security and updates on Windows for PC's they've already sold - it would either never happen or not happen correctly. Swap the players and that's where Android is today.
(yes, I know about alternate firmware - how many non-geeks are capable of using it and have devices that don't have technological blocks like signed bootloaders to prevent it?)