Everyone’s got their own method for estimating this, but honestly it's way more variable than you'd think. The classic go-to is the standard 250 words per page for a typical trade paperback novel, which gets you right around 20 pages. That's for something like a modern genre paperback with decent-sized type and normal margins.
Where it gets messy is when you step outside that. Academic texts or technical manuals? They cram in way more words, sometimes 400-500 a page, so you could be looking at only 10-12 pages. Meanwhile, a middle-grade chapter book with lots of illustrations and bigger font could stretch those same 5000 words to 40 or even 50 pages. I once compared a dense fantasy epic to a children's poetry collection—the word count was similar, but the physical thickness was night and day. So if someone asks, I usually say 'it depends,' and then annoy them by asking what kind of book they're thinking of.
The binding and paper stock play a role too. A mass-market paperback feels denser than a trade with its thicker, creamier paper. It's one of those things you don't really notice until you're holding two books with the same word count and they feel completely different in your hands.