lucy24 - 1:05 am on Aug 7, 2013 (gmt 0)
18 seems like an awful lot, unless your site is an encyclopedia and that's what people expect to see. It isn't enough for the whole TOC to fit on the screen; the user should also see stuff that isn't TOC.
The word count isn't so much of an issue. 4-8 words is comfortably with a single line, with space left over for visuals.
But wait. Is the list meant to be in addition to a horizontal navigation bar? You don't want it to look as if you're offering the user dozens of ways to be somewhere else ... and no reason to stay where they are :)
You could defer your in-page navigation. h1, picture, preliminary blahblah, and then the list. Then there's something to catch the user's attention when they arrive. Once they've decided that the page has something they want, then show them how to get there.
How far apart are your h2s and h3s on the page? Option C is to start with a list giving only the h2s. And then if a particular h2 has a lot of h3s under its wing, it can have a smaller list of its own. Now at this point I'm working strictly by user intuition. But it seems like you want a unified TOC to show things that the user could otherwise only find by scrolling. If you have several h3s visible on a single screen, they don't need to be separately listed.