In my experience ceramic coatings on headlights don't last, a few months max. Well, at least the ones designed for paint. Maybe ceramic coatings designed for trim last longer, but I didn't try those, because those tend to develop a less clear finish - and obviously your headlight is the last place where you would want your coating to haze up or something.
On the other side even if the coatings on the headlight would last for years, it wouldn't really matter, because obviously headlights are the most heavily bombarded parts of the car (well, next to lower front bumper anyway) that are continuously being hit by bugs and dirt, but that are the least tolerated to have dirt and obvious damage on them (because it's more visible on the clear lens, than on any other part or material). Because of this headlights, ceramic coated or not, develop small defects very quickly again - so you will want to lightly polish and recoat them at least once a year anyway.
All in all I personally don't think it makes much sense to ceramic coat the headlights, unless you're coating or re-coating at least some parts of the car anyway. Also, you will waste a lot of applicators and product, if you'll only open the bottle to coat a headlight or two, but not do the rest of the car - so, your question about how many headlights you can get out of a bottle doesn't make much (economical) sense to me in this way.