Read this. Character limits are laid out clear as day. Using a static image as with headline + primary text as an example. https://en-gb.facebook.com/business/ads-guide/update/image
Text recommendations
Primary text: 50-150 characters
Headline: 27 characters
You can technically write longer but it’ll get cut off with ellipsis “…” . Not a great first impression for any business.
Note that primary text shows first for some ad layouts so thats what people might see as the “headline” at the top of an ad. Longer characters allowed there- I used to treat it as the unofficial headline when working at agencies. Fire up a static ad and see the ad preview for what I mean.
It’s often faster googling what you’re after than a reddit post for these kind of technical specification debates you and your boss will have.