Dim FSO As New FileSystemObject
Dim Folder As Folder
Dim File As File
Dim lNummer As Long
 
Set Folder = FSO.GetFolder("c:dein ordner")
For Each File In Folder.Files
  If Val(Mid$(File, 2, 3)) > lNummer And LCase$(Right$(File, 4)) = ".dat" Then
    lNummer = Val(Mid$(File, 2, 3))
  End If
Next
 
MsgBox "Nächste freie Nummer: " & CStr(lNummer + 1)