It's seems to me highly improbably that she really had that big of an army. But how do historians get that sort of numbers? And furthermore can you tell a bit more about her army: the structure, the logistics?
Ultimately, we can't truly know the numbers. We are entirely beholden to the Roman account of the battle and, like most Roman historical accounts, it was a panegyric written with a specific agenda. Further, the Romans often exaggerated their military accounts. However, the Romans killed the keepers of the British oral histories (the Druids) so the Roman perspective is all we have. :/
As for logistics, we're told that they brought their entire families in tow. So it wasn't just an army, it was also a massive baggage train with carts, loved ones, and all manner of supplies traveling with it. Ultimately, we're told in the Roman account, that it was their presence that lead to the utter slaughter of Boudicca's army since they were hemmed in by their own carts and couldn't retreat.