Is someone can help me to write a code to add "RankID" column in my source file?

Sample Source Data
Source_id,sta_id ,Address1 ,City ,PaymentMethod,Invoice_Detail
21 ,KXV ,Rd , , ,
22 ,KXV ,PO Box234 ,Dallas, ,
31 , , , ,Cash ,;8555-4
42 ,Paid Sat , , , ,
Y ,180217 ,6 ,23550 , ,800-345-969
21 ,CNN ,TV , , ,
22 ,CNN ,PO Box 99 ,Chicag, ,
31 , , , ,Cash ,224;86799
42 ,Paid Sun , , , ,
Y ,180218 ,6 ,3456 , ,888-191-1900
Y ,180226 ,7 ,456 , ,800-900-8999 

What I want after add/populate "RankID"

RankID,Source_id,sta_id ,Address1 ,City ,PaymentMethod,Invoice_Detail
1 ,21 ,KXV ,Rd , , ,
1 ,22 ,KXV ,PO Box234 ,Dallas, ,
1 ,31 , , , ,Cash ,;8555-4
1 ,42 ,Paid Sat , , , ,
1 ,Y ,180217 ,6 ,23550 , ,800-345-969
2 ,21 ,CNN ,TV , , ,
2 ,22 ,CNN ,PO Box 99 ,Chicag, ,
2 ,31 , , , ,Cash ,224;86799
2 ,42 ,Paid Sun , , , ,
2 ,Y ,180218 ,6 ,3456 , ,888-191-1900
2 ,Y ,180226 ,7 ,456 , ,800-900-8999 

Thanks in advance.