If you are accepting Legends answers, then Richard's comment stating it is 10,000 troops is your answer.
Outside of Legends there is no clear answer. A few definitions:
- A Roman legion consisted of between 3,000 and 6,000 troops.
- A military or semi-military unit.
- Any large group of armed men.
On the one hand, it is possible that the Emperor is exaggerating or otherwise overstating the actual force on the moon, because fear and despair lead to the Dark Side, and he is trying to convince Luke there is no hope but to join him.
On the other hand, he could easily be referring to an actual legion, because he did predict the rebellion would make an assault on the base. In the military there are specific terms relating to the size and makeup of your troops.
And on the gripping hand, unless specifically stated in some future canon work, we just don't know for sure.