There are tons of other techniques you can use.
For example you can change the code in your header.php so that the horizontal menu is NOT exactly equal to your Pages. Customize the menu. Instead of calling in wp_list_pages()
(which will list out every Page you make whether you want it in your header or not), instead you can just write the custom links you want in the top menu one at a time with <li>
and </li>
separating each hyperlink. Then you might have SOME of your pages in the horizontal menu, and you might link to the other pages somewhere else in your theme.
Another method is to change the CSS for the horizontal menu so that it can go on 2+ lines. Make it wordwrap when it gets to the edge of your theme container.
Yet another method is to modify the Text size in the CSS. Change it to smaller font and more will fit.
Or change the CSS to make less “whitespace” between the menu items.
With the right CSS, you could probably fit about 100 links in that header. Not that you want to. Your options are unlimited.