I have recently changed the number of posts displayed in the wp-panel (from 20 to 999).
My surprise is that after the changing, when trying to see the posts a blank page appears.
How can I come back to see the posts (20 for instance)?
If you are looking to reset "Post_Per_Page" field, please refer attached screenshot, you need to update "Blog pages show at most" file and you can find it at Admin >> Settings >> Reading.
If you are unable to access admin area as well, you can change it directly in database. To change this variable directly in database open "wp_options" table, search for record where "option_name" = 'posts_per_page' and update "option_value" column for that record.