Great question, I'm assuming that your talking about the battle of the Teutoburg forest, it was the worst defeat in roman military history compared with hannibal. Almost 16,000 men died in the battle when they got ambushed by barbarians, it was the betrayal of Arminus the Roman Legatus that betrayed his army. This happened in 9 AD but your probably talking about the entire Roman military army which was roughly, give or take around 160,000 men.
