Pipeline structure help needed

I think I have a solution, just need to work out the syntax.  How could I use the {{n | count}} in a condition to see if it is equal to zero?  If it is zero, that would mean this is the first iteration and I could create the record in the Invoice table and include the invoice number in the add row to upsert.  This would ensure I only create the invoice record once.

------------------------------
Paul Peterson
------------------------------