The shortened version of the thread title, from the main screen, said "How many submissions does ..." I thought it was going to finish "it take to change a light bulb". I'm disappointed now, no joke.
I have no idea of the total received daily, many are weeded out by anti-spamming filters. As an editall "my category" is the whole lot, same for most editors who post here - editalls and metas, and the other regulars are usually sith overlords of one of the branches with anything up to a million sites listed in them. We list circa 2000 sites a day, maybe manually remove twice that many in duplicates and spam, and I haven't noticed the numbers waiting increasing massively.
Most popular categories are those people think they can make money out of being listed in. Often that coincides with the categories editors are least interested in. Combine the two and well you guess ... Think of the type of spam you get in your email - well we get the sites that go with those emails.
Google will index your granny's shopping list if it can see it. We only list sites with quality content, each one after a proper review by a human being (or one of the cats many female editors seem to keep on their laps whilst editing). That's what makes us unique alongside being very cheap, so cheap we give 'em away.