Eight to ten thousand seems very possible and that's a lot of Vikings they have a camp nearby we are told by abbo that they are raping pillaging taking slaves killing everybody in the vicinity he makes them sound over and over again like Tolkien's orcs. Viking attacks on the continent you know against the formerly or current Frankish empire i don't know if that's exactly true but it's pretty safe to say that this is one of the largest groupings of Vikings that the world has ever seen. There may be a clue into the building practices of the early middle ages when we find out that they've been working on the defenses here from for 20 plus years and they're still not totally completed