There is no simple VBA function that will give you this information. A workaround method for a letters merge document would be to merge to a new document and count the sections e.g.
Code:
Sub CountRecipients()
Dim lngCount As Long, i As Long
Dim oDoc As Document
If ActiveDocument.MailMerge.MainDocumentType = wdNotAMergeDocument Then
MsgBox "The active document is not a merge document?", vbCritical
Exit Sub
End If
With ActiveDocument.MailMerge
.Destination = wdSendToNewDocument
.SuppressBlankLines = True
With .DataSource
.FirstRecord = wdDefaultFirstRecord
.LastRecord = wdDefaultLastRecord
End With
.Execute Pause:=False
End With
Set oDoc = ActiveDocument
lngCount = oDoc.Sections.Count - 1
oDoc.Close 0
MsgBox lngCount
End Sub