Easiest way to implement this depends on how you use the repository and metadata, but this is the approach I would take.

Repository Structure

X Department Cases

• 📁 2026
  • 📁 L26-0001
  • 📁 L26-0002
  • . . .
• 📁 2025
  • 📁 L25-0001
  • 📁 L25-0002

Each of the year folders can have template metadata set for the "next available doc number"

Workflow runs and finds the year folder by year

1. current year doesn't exist -> create it and add field "next available doc number" = 1
2. Current year does exist
   1. save "next available doc number" to workflow variable
   2. immediately set "next available doc number" to # plus 1.
      1. small intricacies getting the 0's padding but its trivial
3. create the document's folder named L26-000X in the repository
   1. When creating the document if a LXX-X doc already exists
   2. get the "next available doc number" value again
   3. if it fails again suspend the workflow (you have too many things happening simultaneously
4. Store any related docs under the folder
5. save the doc number as a metadata field for any related docs as well

The year folder structure is useful for the doc numbering you need and for records management purposes if you're using it. You can change the field security of the "next available doc number" field so only workflow/admins can change it.