Depends on how they did it. Shouldn't have to count at all. One way on the old site is to click on the username and view their latest activity. You have to go keep going back through older posts to see everything for a prolific poster. If the OP or their friends went through in a brute force manner, which, I'll admit I kind of assumed he may have done, that could take a lot of time. His answer made it sound like it took his possibly inebriated friends some time to come up with the result. It sounds like he delegated the grunt work to his friends, so maybe he's a manager.
Both sites give you a way to page search a user's posts and page through results, so if you know the page size (may have to count to get it) you can page through and get a count that way. Neither has a way that I see to get a count for all postings in a day without a little work. Not that I'd likely be looking for such an answer.