You can add up to 50 "normal" (customized) search entries (plus Hotclick entries), but it's easier to just replace one of existing 17 entries.
If you don't have a search.ini or an "empty" search.ini, only Google, AllTheWeb?, Super, and Find in page will get listed (that's probably a bug).
Opera won't add other built-in search engines (like Find in page) automatically if you add more than three search engines to your search.ini (that's probably another bug).The list of Search Engines has to be ascending and must not contain any gaps. If the list is incomplete, all search entries after the last correct one will be missing in all dialogs.
If some of your Search Engines are listed in the Hotclick dialog and in Preferences but are not displayed on the Personal bar, it's because of missing Key entries.