Insert sufficient blank records at the start of your data file to
accommodate the missing labels.

The following macro will enter the necessary number of blank records at the
start of the data source:

Macro to set the first label on a part sheet of labels for a label type
mailmerge.
Dim MMMDoc As Document
Dim dsource As Document
Dim dtable As Table
Dim i As Long, j As Long
Set MMMDoc = ActiveDocument
With MMMDoc.MailMerge
   If .MainDocumentType = wdMailingLabels Then
       If .State = wdMainAndDataSource Then
           Set dsource = Documents.Open(.DataSource.Name)
           Set dtable = dsource.Tables(1)
           i = InputBox("Enter the number of labels that have already been
used on the sheet.", "Set Starting Label")
           If IsNumeric(i) Then
               With dtable
                   For j = 1 To i
                       .Rows.Add BeforeRow:=.Rows(2)
                   Next j
               End With
           End If
           .Destination = wdSendToNewDocument
           .Execute
       End If
   End If
End With
dsource.Close wdDoNotSaveChanges

The macro should be run when the mail merge main document has been set up,
and the merge is ready to be executed.  It will execute the merge to a new
document which can then be printed out onto label stock.  The modified data
source is not saved, so that if necessary, it can be used again, commencing
with a full sheet of labels or with a part sheet of which a different number
of labels have been used.