Get reference count by fomula?

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I want to fill increment number in Column “count” by the Column “ref” info.
Column “ref” is Reference and I chocie “A” or “B” or “C”

I want when I choice something in the column “ref” (eg “A” or “C”)
then automatically ‘count’ Column add next number

but how? I found each column has unique $id and same reference field share the $id value.
but I don’t know how to use them

This formula should do the trick, as shown here:

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