Hello! I've been collecting information about the Great Heathen Army of vikings that raided England starting in the year 865, and I'm conflicted abouy a piece of info I've run across. Most sources I've found say that the Army was... well, BIG, the biggest army the vikings had ever amassed, but they tend to avoid giving an actual number. However, the book "The Great Heathen Army: Ivar 'the Boneless' and the Viking Invasion of Britain", by Benjamin James Baillie states that they were 10.000 to 15.000 strong.
Could that really be possible? Seems like a bit exagerated, doesn't it? I mean, I know that the greeks and the romans gathered enormous armies, but that many danes sailing at the same time to raid... What do you think?
First off, while there were víkingar in the GHA, it's likely that the vast majority of warriors weren't actually víkingar, but rather were the warriors who owed allegiance to landowners who had agreed to follow the Ragnarsýnir to England.
That said, 10-15,000 warriors in the GHA isn't an excessive amount when you consider that the Ragnarsýnir (Ivarr, Ubbe, and Sigurðr) were nobility and, more importantly, land-owners in Denmark; this allowed them to recruit not only their own huskarlar/household troops, but hirðmenn from their lands, and convince neighbouring land-owners and minor nobility to join with their huskarlar and hirðmenn, as well as hiring víkingar to supplement their numbers. They probably started with a few thousand men (40 or so ships, with an average of 50 men per ship, works out to 13 ships per Ragnarsson to get two thousand men there - a large number, but certainly not unheard of for wealthy war-leaders in the 9th century) and, with the promise of plunder, more men will find their way to the burgeoning army, and men from across Scandinavia - not just Danes, but Norwegians and Swedes who were looking to get in on the plunder.
Also, keep in mind that's the purpose of the GHA wasn't to raid; it was to invade and settle, thus extracting wealth and land from the Anglo-Saxons on a permanent, rather than temporary, basis.