Structure and Accessibility
Organization may be one of the key points about screenwriting. Without a format that will communicate clearly to the reader they will have trouble visualizing not only the finished film, but also the entire practical production process. This is part of why the screenwriting format is so specific: it requires a common language that everyone expects so they can approach it easily. Part of this structure includes numbering the pages in the correct way so that moments in the script can be easily referenced and pre-production can remain organized once the script begins interpretation.
Page numbers for screenplays always go into the upper right hand corner of each individual page. After this you do a double space below it before you get to the regular bulk of the screenplay text. Often times the first page, title page, and credits page, will not use a page number what so ever. Keep the font, typeface, and character size relative to the one used in your screenplay. Twelve point Times New Roman is always going to be preferable.
When and How
You are going to want to add these page numbers once you are finished with your first draft of the screenplay. The reason for this is that you will likely be moving text around, adding pages, and changing scene lengths. This will then change the location of the page number that you established. Occasionally you can add an extra scene and just make it look like page “40A" if it is additional and parallels page 40. This should not be used often and is just an option in case you were almost completely finished and had to add something spontaneously. Also try to leave this device for the later parts of your screenplay and never in a screenplay under seventy pages.
One of the main reasons for having accurate page numbers is for the script supervisor and director to do scene breakdowns. The general rule of thumb is that a single page translates to about a minute on screen, so your total page number can generally tell you how long your film is going to be. Go through your script once it is finished and make sure that each page is numbered and that the numbers remain sequential. This seems obvious, but in the awkward process of screenwriting it can end up being a problem.