Conceptually, count the relevant rows that precede or include the current row. Here's a draft (and untested) expression for that approach. If the rows are sorted by some column, substitute that for [_RowNumber]. Depending on when you need the index number assigned and whether it might ever change, the expression might belong in the column's Initial value or App formula property. If the former, the expression might benefit from a modified approach; if the latter, the column might be either in the data source or virtual.
COUNT(SELECT(pag2[id_pag1], AND([id_pag1] = [_THISROW].[id_pag1], [_RowNumber] <= [_THISROW].[_RowNumber])))
There are likely more elegant methods. Beware the potential drag on app performance.
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