find out of sequence groups of numbers using vb6 in a listbox

The data you show doesn't seem to match your description; there are 11 of 10001000819 and 13 of 10001000821, so that's probably a typo, but there are 5 10001000823.

What do you want to do when you find that there's a group of 12 or 4?

yes --- so the vb program would need to identify this as not correct. it needs to look for 12 x 12 x 12 … x4 and if not  then highlight maybe the incorrect ones. or somehow id sequence out  of order.
here would be a correct one  (maybe)

10001000792
10001000792
10001000792
10001000792
10001000792
10001000792
10001000792
10001000792
10001000792
10001000792
10001000792
10001000792
10001000793
10001000793
10001000793
10001000793
10001000793
10001000793
10001000793
10001000793
10001000793
10001000793
10001000793
10001000793
10001000794
10001000794
10001000794
10001000794
10001000794
10001000794
10001000794
10001000794
10001000794
10001000794
10001000794
10001000794
10001000795
10001000795
10001000795
10001000795

Would a mixture of just 12s and 4s be correct?

no it would start off as 12 and stay 12 until 4. (4 at the end)

here is what a co-worker came up with in excel but i need it in vb

=IF(AND(A2<>A1,A2<>""),IF(A6<>"",IF(COUNTIF(A2:A13,A2)=12,"","Error"),IF(COUNTIF(A2:A5,A2)=4,"","Error")),"")

Try this code. Note that you should set List1's Sorted property to True in design mode.

Private Sub Command1_Click()
    Dim intIndex As Integer
    Dim strOld As String
    Dim intCount As Integer
    Dim strError As String
    
    strOld = List1.List(0)
    
    For intIndex = 0 To List1.ListCount - 1
        If List1.List(intIndex) = strOld Then
            intCount = intCount + 1
        Else
            If intCount = 12 Or intCount = 4 Then
                ' It's OK
            Else
                If strError = "" Then
                    strError = List1.List(intIndex - 1)
                Else
                    strError = strError & ", " & List1.List(intIndex - 1)
                End If
                intCount = 1
                strOld = List1.List(intIndex)
            End If
        End If
    Next
    
    ' Validate that the last 4 (and no more) are the same
    With List1
        If .List(.ListCount - 1) = .List(.ListCount - 2) And _
           .List(.ListCount - 1) = .List(.ListCount - 3) And _
           .List(.ListCount - 1) = .List(.ListCount - 4) And _
           .List(.ListCount - 1) <> .List(.ListCount - 5) Then
           ' OK
        Else
            If strError = "" Then
                strError = .List(.ListCount - 1)
            Else
                strError = strError & ", " & .List(.ListCount - 1)
            End If
        End If
    End With
    
    If strError = "" Then
        MsgBox "Valid List", , "List Validation"
    Else
        MsgBox "incorrect count found in " & strError, , "List Validation"
    End If
End Sub

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it will always be 12 x 12 x 12 x …..  x 4
if not then that is considered an error  and the groups would always be together (if not then error)

My latest code should be OK then.