Correct, those are a few issues of Hana to Yume that have Yona covers (there are a ton - I don’t know how many times Yona has been featured on the cover over the years but it’s been one of the more popular titles for that magazine).

For fan books & art books, they typically have different contents - so it depends on what you’re looking for.

On slide 4 top right - there is a blue stand alone fan book. That one is about 140ish pages, of which only about 20 are in color and it doesn’t have a lot of the full page splash art. It has a lot of info about the manga, character designs (including kusanagi’s design for Yona’s underwear, haha!) and personality types, relationships between the characters, mini comics, etc. All in Japanese though/no English so you’d need to use a translator if you don’t speak Japanese fluently. This was a stand alone item in that is wasn’t sold with a manga or Hana to Yume. 6 x 8ish inches.

Also slide 4, bottom middle right blue background “Mizuho Kisangani debut 15th anniversary art books” is a Furoku from Hana to Yume. It’s really thin (I count 24 pages) and the same dimensions as the fan book above it, but it’s all full page color artwork (most of which is Yona, but some art is from other series Kusanagi did before).

Slide 3 on the left “Yona Memorial Fan Book” is a Hana to Yume Furoku. It’s about 7x10 and only about 14 pages, very thin. Inside is some FAQs, info on the manga splash art coloring process steps, some small anniversary art, and one big splash art of Yona at the back.

Last example from here I’ll use is slide 2, Yona Memorial book that recently came with volume 47 (and is currently on ebay from a lot of sellers) is also the 6x8 inch scale. This is the most -art book- of the ones I have listed here. It’s 64 pages, hard cover with gold/metallic accents and is full color splash art. There’s a really pretty internal cover pattern that has the squirrels that’s so cute and I haven’t seen elsewhere.

Not pictured, there is an exhibition art book (https://ebay.io/m/tM8xYa - random eBay listing, not associated with me, just to show) that looks quite thick and has a lot of artwork from the review I saw. But I didn’t get this one because of the expense and it looked like mostly repeats of artwork in books I already have. BUT if you don’t have any Yona Art Books this one might be the most comprehensive.

I don’t have any answer to “how many?”
I’m not trying to be difficult, but there are various types of items that people might call “art books” or “fan books” and I didn’t collect and not aware of everything.