Another answer:
I do not know if the above answer is correct or not, but I added a Related Link below to a picture from a scene of the Great Hall at Hogwarts at Christmas. It looks like 5 total, with four on the sides and one large at center, but this camera angle might be misleading.
Update - I found a website that stated from the Sorcerers Stone: "At Hogwarts, the Great Hall was decorated with twelve large Christmas trees brought in by Hagrid and decorated by Professors McGonagall and Flitwick in various ways. A tremendous feast was held in the Great Hall on Christmas day.".
