add an identifier in column

I want to add another identifier in column M, that should count the rows, e.g., the first row in that file has one value in "old", that is “0",
the second datapoint(from row 3 to 7) is instead formed by data of 5 rows (subset) and same value in "old" coloumn, so i can call tag those group in column M as “1", and so on. My question is how can i create this type of identifier and tag in excel. screenshot attached below. Thanks in advance.

Hello and welcome to the forum

Please upload a workbook or a representative cut down copy, anonymised if necessary. It is always easier to advise if we can see your request in its context.

Show a before and after situation with manually calculated results, explaining which information is data and which is results, and if it's not blindingly obvious how you have arrived at your results some explanatory notes as well.

To upload a file click the Go Advanced button at the foot of your post, look underneath the post area for the Manage Attachments section and take it from there.

Why do you include rows 17:19 in the same block as rows 8:16. The values in the Set column are different

Perhaps with a 1 in M2, in M3 copied down

=IF(H3<>H2,M2+1,M2)