So far, I have understood that your requirement is to find a number in csv and then write in some values in that csv itself?
Please correct me if that’s not the case and provide requirement clearly.
Any case, to check whether a particular number contains in a csv or not, you can instead use activity - Read csv
And save its output in a datatable.
Then check whether that datatable contains a number you are looking for. If yes, follow the steps needed.
You can refer below links for more info in this activity.
Hope that helps.
Regards
Sonali