Power Query - How to count the rows and display it as a column?

Hi All,

In "Source" column, I have three different category/ options i.e. STK, SBO HFM.  Based on these cateogry I need to add a new column where it counts SBO by SBO and HFM by HFM and STK by STK and display it as a sequence column e.g.

SBO      1

SBO      2

SBO      3

SBO      4

HFM     1

HFM     2

HFM     3

HFM     4

STK       1

STK       2

STK       3  

Please help out! Cheers,