Python pandas how to groupby column and perform count function without remove any duplicate row?

Question

I have following table:

  id  Key  series_no
  0    B      a3
  1    B      t7
  2    A      u3
  3    A      p8
  2    A      7u
  1    B      zz

I would like to perform count function at column Key that will groupby id column. Below is my expected output

  id  Key  series_no   count
  0    B      a3        1
  1    B      t7        2
  2    A      u3        2
  3    A      p8        1
  2    A      7u        2
  1    B      zz        2

I have try below code but does not work for me:

 df["count"]=df["Key"].groupby(["id","Key"]).count()

Anyone have ideas?