I think it is easier to deal with Content Controls so I would do this with the following code after putting a Content Control titled "CertNumber" in the location you want the number to appear.
Code:
Sub JHA_Print()
Dim iStart As Integer, iEnd As Integer, i As Integer, aCC As ContentControl, sVal As String
'On Error GoTo ErrHandler
Set aCC = ActiveDocument.SelectContentControlsByTitle("CertNumber")(1)
sVal = aCC.Range.Text
If Not IsNumeric(sVal) Then
iStart = 1
Else
iStart = CInt(sVal) + 1
End If
iEnd = CInt(InputBox("What is the last Certificate to print?", "Print Certificates To", iStart + 1))
If iStart > iEnd Then Exit Sub
With ActiveDocument
For i = iStart To iEnd
aCC.Range.Text = i
.PrintOut
Next
End With
'ErrHandler:
End Sub