Is there a way to auto-increment a number field so that the numbers only happen within the subtable record of a parent record?

OK, that's probably not clear, so let me use an example of what I want to do (this is not my use case which may be to obscure, but it illustrates the point:)

Say you have a main table called Contacts and a sub-table of that called "interactions". For any particular contact, I want to be able to auto-increment the number of interactions. When I use the code suggested in the Ninox manual (for Invoices) it auto increments for all 'interactions' en masse. Meaning  if contact 1 has 3 interactions then I add an interaction for contact 2, it automatically increments that to 4. I want the numbers to be specific to each contact. Clearly the code needs to reference both the parent and sub-tables, but I don't know how to do that.

Does this make sense? Any thoughts