An SEO audit should not be rushed. It simply takes time to uncover root causes of the issues affecting your online health. Depending on the size of your site, a proper audit can take anywhere from 2-6 weeks to complete. Due diligence is required when making major changes to any website, and an SEO specialist must conduct a thorough investigation to make accurate, impactful recommendations.
There are other parts of SEO which you should pay attention to after your audit to make sure you stay competitive. After all, the technical foundation isn't the end of the road for SEO success. It's important to pay attention to your competition's SEO activity, keep an eye on the newest search engine best practices, and maintain local SEO best practices if your business depends on customers visiting a physical address. All of these are elements of a successful SEO strategy and should be corollary to your audit and ongoing SEO maintenance.
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