In most cases, if you import a properly formatted Final Draft script (.fdx), it should automatically populate the Page Numbers. You can also update page counts on your scene strips at any point.

However, if you imported a long script and the page count didn't appear, there's a chance that when you converted your script to FDX, the converter (WriteDuet) doesn't properly convert the page length metadata. So as a next step, you can either:

Open your script in Final Draft or Highland and immediately save the file. This restores the page length metadata that was lost during conversion.

Add pages manually by editing each strip. Just click the "more options" button on far right and select "Edit Scene." You can also click a strip and type "e" (hotkey) to pull up the edit screen.