Thanks for your quick response

The file creates a serial with two criteria in column F, the first criteria is Item Code and the second is PO # (I did this manually)

I made an attempt with the equations in column J but it failed

I would like to make an equation in column F that gives the results in it

If you accept helper columns as per your attachment.

Then, try this,

1] Helper column_1, H3 formula copied down:

=IF(COUNTIFS(B$3:B3,B3,C$3:C3,C3)=1,IF(COUNTIFS(C$3:C3,C3)=1,MAX(H$2:H2)+1,0),"")

2] Helper column_2, I3 formula copied down:

=IF(H3=0,MAX(I$2:I2)+1,0)

3] Formula result, J3 formula copied down:

=IF(H3="","",H3+I3)