Choosing the correct category can also help swing things in your favor as well. For example, just about everyone enters something into the single Warhammer 40,000 model category, where as the Duel category gets significantly less entries. At least from what I've read online.

There is nothing wrong with entering any category you wish, but it can never hurt to tip the scales to your favour every now and then.

Well, that's my secret busted!! you'll all be in duel with me now 😩

Yup, tactical category selection can be key. But it's not just about the sheer number of entries, it is also about the quality of the best in each class and the barriers to entry .. I.e. The amount of time required to complete an entry for a class. Most people have a single mini somewhere that they are proud of, so entering single mini categories has a low barrier, so this swells the number of entries. It was far worse in the Games Day days when you could get into the event an hour early if you entered GD, so loads of people brought single minis just to get in early. I even remember an empty flocked base brought in as Frodo wearing the ring! The top painters will spend 100 or even 200 hours painting a single mini entry, so enter those at your peril if you are starting out. That said the Lord of the Rings category does ebb and flow in quality as the mini releases are not as appealing to painters as 40k and AoS, so if you want to paint a single mini, go there.
To be honest, all of the other categories vary in standard from year to year. The unit classes tend to look fuller because most gamers have a good unit they game with so can bring that along. However, units and larger models take more time to paint, so if you are a person who spends 150 hours on a single mini, a unit of ten to the standard you desire is a big ask. The top bods do enter these categories but in lower numbers The large model categories have been made harder to enter too as you can no longer enter an ogre or an ork boss (I was guilty of both before the rule change 😉). But if you can deliver a very consistent paint job, perhaps with a few extra twists added to a model, you can do very well in the unit and large model categories. Actually Bloodbowl teams and necromunda gang categories are good possibilities as not everyone wants to paint them. Certainly Bloodbowl appeals to fewer people.
And yes, my beloved duel category does also suffer from a shortage of entries at times, more so at the mini demons because people have not had the time to put into creating something (I can spend nearly as much time building a duel as I do painting it). Diorama is also often short of entries. With those two categories, you have to do more than just build a model and paint it, so a barrier to some. I'm actually a diorama builder by inclination, but they take too long so a duel is the next best thing. Getting some real action going on between two model is vital in duel. You can't just plonk two models on a base facing each other. However, getting something going on is easier now as there are more action poses available, but you still need to do something extra to set a scene etc. The two duels that I am painting at the moment have no conversions at all,just mini kit bashes, whereas for previous ones I have had to do some serious kit bashing and converting to get some decent poses.
The one thing that I would warn against is spotting weak categories from an event and going for that the next year. Everyone else will have done the same 😂. (I also used to show dogs, and the same happened there). I remember a weak duel year a while back. The following year it was inundated and the standard was too high for me to live with. The double bluff might be best. Look for the scariest category (outside of 40k and AoS single) and go for that. But now you are wondering if everyone will do that, so should I now go for the triple bluff..... 😂😂😂.
Or a good tactic could simply be to stick to one category over a few years and hope that the ebb and flow of the standard of entries falls in your favour from time to time. I have been doing that with duel for a long time but have also had runs in monster and unit categories.
Perhaps most importantly, make sure that you are painting something that you want to paint. You are going to spend a lot of time doing it, and it is supposed to be fun!