A counter $(count) only counts the number of use of the command, and can’t be used to count how many times a specific user has been slapped.
A way to go about this would be to make your own API and call it with Nightbot.
That said, I’ve worked on solutions in the past using an already existing API, but we don’t recommend doing this as it goes against how the dev intended their API to be used, so you’re on your own if you want to adapt my code to your needs: