Sub Test()

Dim i As Long, x As Long, y As Long, initials As String, numberofsquares As String

initials = Application.InputBox("What are the initials?")
numberofsquares = Application.InputBox("How many squares?")

Do Until i = numberofsquares
    x = Application.WorksheetFunction.RandBetween(2, 20)
    y = Application.WorksheetFunction.RandBetween(4, 8)
    
    If Cells(x, y).Value = "" Then
        Cells(x, y).Value = initials
        i = i + 1
    End If
Loop

End Sub