This version will highlight each paragraph that contains a 1,000th word. It is more accurate on what it thinks a Word is.
Code:
Sub TagEvery1000thWordParagraph()
Dim DocSrc As Document, lCount As Long, lDocLength As Long, aRng As Range
Set DocSrc = ActiveDocument
DocSrc.Range.HighlightColorIndex = wdNoHighlight
lDocLength = DocSrc.Range.End - 2
Set aRng = DocSrc.Paragraphs(1).Range
Do Until aRng.End > lDocLength
lCount = lCount + 1000
Do Until aRng.ComputeStatistics(wdStatisticWords) > lCount Or aRng.End > lDocLength
aRng.MoveEnd Unit:=wdParagraph, Count:=1
Loop
aRng.Paragraphs.Last.Range.HighlightColorIndex = wdRed
Loop
aRng.Paragraphs.Last.Range.HighlightColorIndex = wdNoHighlight 'remove the last para highlight
End Sub