style - Should I use letters or digits in the following case?

De-Shi was holding something that looked like a price tag. It had the numbers 024 written on it.

"Isn't that what cows wear as earrings?"

"Yeah," De-Shi said, still inspecting the object, a thoughtful expression on his face, "an ear tags."

I knelt down next to De-Shi. “024. I think it belonged to the cow number 24. You know, from the 30 that jumped.”

“It’s possible.”

Right now I'm only using digits. Should I change some of them to letters? (I always get confused about this. Is there a general rule that tells you when to use letters and when to use digits?)

Answer

I'd do:

De-Shi was holding something that looked like a price tag. It had the numbers "024" written on it.

(1) quote any text, no matter if it was spoken or found written somewhere
(2) if you don't quote the numbers, 024 is one number (not plural), so either 'the numbers "024"' (read: oh-two-four) or 'the number 024' (read: twentyfour)

"Isn't that what cows wear as earrings?"

"Yeah," De-Shi said, still inspecting the object, a thoughtful expression on his face. "An ear tag."

(3) "tag", not "tags"
(4) I would also split that sentence: '... face. "An ...'

I knelt down next to De-Shi. “Twentyfor. I think it belonged to cow number twentyfour. You know, from the thirty that jumped.”

(5) or: "Zero two four. I think it belonged to cow number twentyfour. ..."

“It’s possible.”