Way to limit # of posts per category posted (not shown)

Hi there,

I’m looking for a way to limit how many posts can be added to a sub-category after which, if someone tried to post one more than allowed, would be greeted with a message or redirect to a page explaining why posts are limited in that category.

It’s an attempt at hacking a directory theme/plugin to make it so that I can limit the absolute number of listings in each sub-category. I know how to limit how many are shown in each category but I want to limit actual postings full-stop before they even get a chance to be added to the database.

I’m in over my head on this one. Can someone help get me started (or, if you know of a plugin that might work)?