I wouldn't put it at above 450 people, including teachers and ghosts.

I'll go witht eh idea that Harry's class is abnormally small, and that there was a baby boom the year after and onwards.

Add in a hunred or 2 hundred house elves, and you're set.

BTW: did you guys notice something in the link to JK's map that MadEyes or someone posted?

HOGWARTS IS BUILT AROUND THE LOCH NESS LAKE!!! Apparently, the giant squid is supposed to be Nessie...she wrote Loch lake on the shitty little drawing. And, there's supposed to be a cliff by the lake.

 

Wrong. Nessie, according to Rowling, is a Kelpie whose favorite form is a sea serpent that turns into a otter when a group of muggle researchers come by. And all lakes in Scotland are called Lochs it could also be an arm of the sea like a fjord.