Sub CopyRandomRows()

    Sheets("Random Sample").Select
    Cells.Select
    Range("C14").Activate
    Selection.Delete Shift:=xlUp

    Windows("Critical Infotype Raw Data.xlsx").Activate
    Rows("1:1").Select
    Selection.Copy
    Application.CutCopyMode = False
    Selection.Copy
    Windows("Critical Infotype Sampling Tool.xlsm").Activate
    Sheets("Random Sample").Select
    Rows("1:1").Select
    ActiveSheet.Paste

    Dim source As Range, target As Range, randCount&, data(), value, r&, rr&, c&

    ' this defines the source to take the data
    Set source = Workbooks("Critical Infotype Raw Data.xlsx").Worksheets("Sheet1").Range("A2:L5215")

    ' this defines the target to paste the data
    Set target = Workbooks("Critical Infotype Sampling Tool.xlsm").Worksheets("Random Sample").Range("A2")

    ' this defines the number of rows to generate based on the input in textbox
    randCount = Worksheets("Main").TextBox1.value

    ' this load the data in an array
    data = source.value

    'this shuffle the rows
    For r = 1 To randCount
        rr = 1 + Math.Round(VBA.rnd * (UBound(data) - 1))
        For c = 1 To UBound(data, 2)
            value = data(r, c)
            data(r, c) = data(rr, c)
            data(rr, c) = value
        Next
    Next

    ' this writes the data to the target
    target.Resize(randCount, UBound(data, 2)) = data

     MsgBox "Random Sample Generated!"

End Sub

' this defines the source to take the data
Set source = Workbooks("Critical Infotype Raw Data.xlsx").Worksheets("Sheet1").Range("A2:L5215")

With Workbooks("Critical Infotype Raw Data.xlsx").Worksheets("Sheet1")
    Set Source = .Range("A1:" & .Cells(.Cells(.Rows.Count, 1).End(xlUp).Row, .Cells(1, .Columns.Count).End(xlToLeft).Column).Address)
End With