How to count rows with specific value in Power Query

Not sure I understand. Your Row Count/Table Select should be after you have this other custom column created. So look at this code:

let
  Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSlSK1YlWSgKTyWAyBUymgsm0ovx0MIOAMjCZnp+ThsRFI2MB", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Product = _t]),
  AddNewColumn =
    Table.AddColumn(
      Source,
      "New Column",
      each if [Product] = "a" then "f" else [Product]
    ),
  #"Added custom" = 
    Table.AddColumn(
        AddNewColumn,
        "Custom", 
        each 
        Table.RowCount(Table.SelectRows(AddNewColumn, each Text.Contains([New Column], "f")))
      )
in
  #"Added custom"

The #"Added Custom" column where the Row Count and Table Select Rows is happening is referencing the AddNewColumn staep above where a new column was created that changes the "a" to an "f"

So the count goes from 7 to 9.

Please post more than just a formula for your custom column. You'll notice I am posting the code from the advanced editor. We have no idea what steps are before and after your custom column function, and that knowledge is critical to building it correctly.