The death toll at the Teutoburgerwald is extremely difficult to calculate. We know that the Pannonian and Illyrian wars had stretched the Roman army to its limits, with at least five legions being directly involved. There is no direct evidence, however, that any units from the XVII, XVIII or XIX were involved, though some may have been.

It's true that legions were often under strength, but this was also a recently conquered frontier province - the idea that these legions had been allowed to run down to half-strength given the importance of this frontier is, perhaps, going a little far. The impact of the loss of these legions would possibly have been far less if they were only at half-strength and it is also something at least one of the sources may have mentioned as a reason for the Germanic victory.

And we can also add the civilians who were accompanying Varus back towards the Lippe and the Rhine - merchants, colonists, camp followers, women and children.

I'd say something closer to 20,000 casualties would be closer to the mark, but it really is impossible to tell.