autofill to variable destination

I have the following code that was achieved using the macro recorder. The only problem I am running into is that during the autofill command, there are never the same amount of rows to autofill. It could be as few as 5 rows and as many as there are rows on the spreadsheet. When I recorded the macro there were 953 lines, and unfortunately I cannot figure out how to change the specific destination of 953 to a variable destination determined by when data ends.

Here is the code I currently have:

Sub ConcatenateHighlight()
'
' ConcatenateHighlight Macro
'

'
    Columns("C:C").Select
    Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
    Range("C1").Select
    ActiveCell.FormulaR1C1 = "=CONCATENATE(RC[-2],RC[-1])"
    Range("C1").Select
    Selection.AutoFill Destination:=Range("C1:C953")
    Range("C1:C953").Select
    Range("F1").Select
    ActiveCell.FormulaR1C1 = "=CONCATENATE(RC[-2],RC[-1])"
    Range("F1").Select
    Selection.AutoFill Destination:=Range("F1:F953")
    Range("F1:F953").Select
    Range("C:C,F:F").Select
    Range("F1").Activate
    Selection.FormatConditions.AddUniqueValues
    Selection.FormatConditions(Selection.FormatConditions.Count).SetFirstPriority
    Selection.FormatConditions(1).DupeUnique = xlDuplicate
    With Selection.FormatConditions(1).Interior
        .PatternColorIndex = xlAutomatic
        .Color = 65535
        .TintAndShade = 0
    End With
    Selection.FormatConditions(1).StopIfTrue = False
    Range("D1").Select
End Sub

Thanks!

Try this:

Sub ConcatenateHighlight()
    Dim nRow As Long
    
    nRow = Cells(Rows.Count, "C").End(xlUp).Row
    
    Columns("C").Insert
    With Range("(C:C, F:F) 1:" & nRow)
        .FormulaR1C1 = "=RC[-2] & RC[-1]"
        .FormatConditions.AddUniqueValues
        .FormatConditions(Selection.FormatConditions.Count).SetFirstPriority
        .FormatConditions(1).DupeUnique = xlDuplicate
        With .FormatConditions(1).Interior
            .PatternColorIndex = xlAutomatic
            .Color = 65535
            .TintAndShade = 0
        End With
        .FormatConditions(1).StopIfTrue = False
    End With
End Sub

Hmm. Edited, try again.