There is no exact number for page authority, It depends on the niche market, In some niche market PA 30 may be enough to rank on top of Google and in some highly competitive markets maybe PA 50 score is best. But there is a practice you can follow to know the best PA score in any niche. List down all of your first page competitors, check page authority of all of them, scroll down to their results, and calculate the average score of all the results.
This will give you the idea of how strong your page’s score is and how much more effort should you put in. The higher the page authority score, the more is the capacity of a particular page to rank in search engine results, especially on Google.