Hej Mats,
If it was file "numbering" what you were looking for, that you can do with automatic layout IDs.
When I used the "split drawing among multiple layouts" the last time, I had a folder with customized ID and Prefixes for the items in the subset.
Then, the layouts within this folder just used the "Automatic Book and Subset ID assignment" under Layout Settings / Identification and Format.
I'm not sure if you can make the filename correspond to the contents of your drawing - for example filename "D21a" for a door with such ID, but you can get something like "001 Doors" or like in my case filenames "A-42-4-1001, -1002, ...". Or you can even get something like "Door schedule 1 out of 30" if you get a little creative.
/Martin