HI,

This ultimately depends on the way your dev team handles work.

With that said, you can break it up so that you can have a repo set up in your Git repository for each developer, a certain aspect in ITSM like one for Change forms, Change Business rules, etc., or however you want to break up with for your team where it is effective of managing. 

If you are using Source Control, the key here is to break up a big global app to have the freedom to get away from the "One branch per application per instance" using source control. Remember that you will still require a repo in the Git repository for each Global App Bundle if you use Source Control.

The beauty of using a Global App Bundle is the ability to "claim" different objects into another Global App Bundle. So if one approach, for example, if you use one Global App Bundle per developer, is not working out. You can restructure to something like a Business Rule Global app bundle and claim all business rules from the old Developer Global Apps Bundle into the new Business Rule Global App Bundle. 

I have seen where they have it broken down by a developer global app bundle and where it is broken out per functionality inside a product like forms, tables, scripts, etc. It comes down to your choice of managing the changes on the platform.