サンプルマクロは、C:\内の複数のブックをランダムに３分の１の
確率で開きます。

Sub Sample()

    'ファイルの存在するフォルダ
    Const myDir As String = "C:\"
    
    Dim myFileName As String
    Dim myCnt As Long, i As Long
    
    '開く確率 = 1 / myRndRate
    Const myRndRate As Long = 3
    
    myFileName = Dir(myDir & "*.xls")
    
    With Application
        .DisplayAlerts = False
    While myFileName <> vbNullString
        Randomize
        If Int(Rnd * myRndRate) + 1 = 1 Then _
            Workbooks.Open myDir & myFileName
        myFileName = Dir()
    Wend
        .DisplayAlerts = True
    End With

End Sub