How to use a running number / identifier across multiple filed

Hallo everyone

I am a new user, so please excuse me if I ask stupid questions. I didn’t find this one by searching, so I hope you can help me.

I have a large file with thousands of entries that are identified with a unique identifier. Each month new Data sets get added in.

I have just programmed a way to aggregate the data from different CSV files.

How can I merge the data, and make sure that the running number/unique identifier is used continuously on the new data?