Ask the children who will be using it what they prefer, and how high they want them.

Because if it's not easy for them, they will toss the backpacks on the bottom of the cubby and drop the jacket on top of it,

The more hooks the better as far as i'm concerned. Our new house has a mudroom, which I love, but just for my daughter alone with her dance bag, summer pool bag, backpack and hoodie, she's already used up half of the hooks, and it's not even winter gear season yet. Plus, my kids are constantly losing their uniform belts so now I have them keep them with their backpacks. I'm planning to add 7 more, and if I could I would do more than that, but I don't think it will look right with the molding (although really all of the pretty detail will be covered up anyway) so some of them will be double hooks. I think you're smart to do as many as you can fit.

My 9 year old says our hooks are too high, and leaves his backpack on the bench. He hangs everything else, I think he just can't heft his heavy backpack up there. But personally, as long as it's neat, I don't care if the backpacks are hung as long as everything else is, and everything isn't spilling out onto the floor. Are your kids tall? If you need to hang them lower, you'll end have empty space at the top of the cubbies, but you could be creative with that.

If I had the room I would make our low cubbies/drawers for backpacks, and they are deep enough, but I don't have space to move shoes anywhere else. I like that your drawers are nice and wide!

I agree w/ elizabetheva's son, and think backpacks are way too heavy to lift onto hooks; they should go on the floor of the cubbies, in my opinion. Just because they have a loop doesn't mean you should hang them.
Plus my kid used to need to get stuff (books and notebooks for homework, etc.) in and out of them throughout the evening, so being able to access them easily is important--and you can't do that when the opening is over your head.

I like the idea of lots of hooks--sometimes an empty, unused hook can get in the way of stuff hanging on a higher or adjacent hook, but it's not that common.

I might have the kid stand there with their actual coat and see how high up is sensible to both reach, and for the coat to dangle.

And I would probably stagger heights and alignment of multiple hooks on a single surface.

If you'll have a few small things (umbrellas, dog leashes), you might consider multiple hooks on the side.

AND...don't forget the idea of wall pockets or wall baskets for stuff like gloves.

ooh, ooh--I had an idea.

You could try out hook placement with Command hooks. And try out stuff like wall pockets that way too (I have a Command hook on the wall in the bathroom that holds a shower caddy, which creates shelves/pockets).

Then you can adjust easily if you've guessed wrong.

Here's one of my favorite wall baskets. It's for a blood-pressure cuff in the doctor's office, but it's a really useful shape and size. (I saw one used in a Martha Stewart Living laundry-room setup.)

https://www.walmart.com/ip/Sphygmomanometers-Inflation-System-Wall-Baskets-MDS9472/45880287

This one is black, but I'm sure they come in white and gray.

And of course, a shower caddy for hanging on the shower head would probably work too.

Here's another interesting size/shape of wall basket from Wal-mart--another medical one.
https://www.walmart.com/ip/R-B-Wire-2255-15-inch-L-x-7-inch-W-x-3.-5-inch-D-Metal-Medical-Storage-Basket-Wall-Mounted/25520268

These are expensive but fancier
https://www.houzz.com/products/hoyle-wire-baskets-2-piece-set-prvw-vr~59246808

Oh, and remember: kids get taller, sometimes pretty quickly. So if you put a row of hooks lower, you might want to add higher ones later.

You could put small pockets or a ledge of some sort above the hooks, esp. if they're a bit low. For stuff like sunscreen.

I really like this and it gave me an idea for my own mudroom. I am going to use sliding hooks like my mother has.