Plex doesn’t know about disc sets.
It only knows movies or tv shows.
Look up Astro Boy on TheTVDB
https://www.thetvdb.com/series/astro-boy
If the show was aired first in 1963, then you must use (1963) in the folder name. (It is much more important there than in the file names, btw)
Use a folder structure which follows the Plex recommendation for TV shows: https://support.plex.tv/articles/200220687-naming-series-season-based-tv-shows/
Use a ripping software which allows you to split the episodes into separate files.
If your multi-episode files have at least chapter markers at the episode boundaries, you can use these to split the files yourself. [HowTo]: splitting multi-episode files with MKVtoolnix GUI
