Number that increments based on a condition

This has been asked a couple of times with slightly different solutions, so seeing current thinking.

I want to have an incrementing number that is specific to a group of records only. In fact I want 3. This is used to create unique names (don’t ask me what this client wants this!), but the number should increment if the record has a field set to x. So when grouped together, there is a sequential incrementation (1, 2, 3, 4,) , but others not in that group do not get that number. They would get a different increment (different field probably).

My current thinking is to link the records to another table (using deep match?), identify the highest number of increment already, and then have an automation add to the new record ‘number +1’. 

Thoughts?