You’re not limited to just one, but you can’t specify a folder. As the text below the Movie Pre-Roll Video field explains, you can specify multiple complete paths to individual pre-roll videos. If you separate them with commas, Plex will play every specified pre-roll video before the movie. If you separate them with semicolons, Plex will randomly play one of them before the movie.
example:
comma-separated - Plex will play all three in order:
C:\prerolls\video1.mkv,C:\prerolls\video2.mkv,C:\prerolls\video3.mkv
semicolon-separated - Plex will randomly play one:
C:\prerolls\video1.mkv;C:\prerolls\video2.mkv;C:\prerolls\video3.mkv
This is obviously not ideal. Besides being extremely clunky, there’s a limit to the size of the field, though I don’t recall what the limit is.
There is a long-standing feature request for exactly what you’re requesting:
Pre-roll FOLDER instead of typing out each path
