When optimising a title, you are looking to rank for as many terms as possible, without keyword stuffing your title. Often, the best bet is to optimise for a particular phrase (or phrases) – and take a more long-tail approach. Note that too many page titles and not enough actual page text per page could lead to doorway page type situations. A highly relevant unique page title is no longer enough to float a page with thin content. Google cares WAY too much about the page text content these days to let a good title hold up a thin page on most sites.
The audit is for all pages and not only one. What happens in the majority of the cases is that pages / posts have similarities so you can group them together. For example the pages of a website may be ok but the blog post pages may be missing titles. It’s a lot of work especially for a 500 pages website but you can start from the most important pages first and work your way to the rest