Finding how many child records for a given parent record

Hi everyone,

In this table, I have 2 projects, CAPAs and Extensions. Extensions are child records of CAPAs.
I need a formula in column Q that counts how many Extensions does each CAPA have, based on the ID column A. Column A has the parent IDs and child IDs. So in column Q, only CAPAs will have a corresponding number that it can be 0, 1, 2, etc, depending on how many child records belong to the CAPA.

I attached the file as well.

I hope I clearly explained what I need.

Thank you guys

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