Surprisingly, I've found that someone has already done this calculation and has concluded that there are approximately 280 students at Hogwarts, with about 10-15 faculty teaching them.

- quote (http://cinemaroll.com/cinemarolling/the-size-of-hogwarts/ ) -
I was thinking after watching the movie, “How big really is Hogwarts?” I realized that each house only has one tower of Hogwarts to house its students in, and I also realized that the dining hall really is not that big. Then, I also concluded that the Potions class is pretty representative of the average class size at Hogwarts and that no students from either Gryffindor or Slytherin (the two featured houses) would really be skipping out; Slytherins would all sign up for the class because it was taught by Snape, and Harry and Ron were the only two Gryffindors who looked like they weren’t there at the start. The featured class really only looked like 20 kids; that might even be a bit generous. With equal numbers of Gryffindors and Slytherins, that means 10 sixth-year students from each house. If all years are equal, then each house has 70 students, giving Hogwarts 280 students.
- end quote -

A discussion on http://harrypotter.wikia.com/wiki/Forum:Anyone_know_the_population_of_hogwarts%3F mentions the JK Rowling claimed there were about 1000 students at Hogwarts but agree that this is probably higher than reasonable., except that perhaps the school never has all the students in the dining hall at once (which at least for some scenes would not seem reasonable). Thus, the number may be larger than ~280, but probably cannot be anywhere near the ~1000 quoted.

With no special reason to expect that the number of wizards should fluctuate from year to year by very much, and with no indication of anyone flunking out, I'd say that class sizes (i.e. the number of students in each year) is probably fairly consistent. If the typical number is 40, then I'd expect the fluctuations to be +/-sqrt(40) = +/-6 or 7.

Since there was never any mention of one or another House suffering a disadvantage due to drawing on a smaller population of students for quidditch, I'd expect the four houses to have similar size populations.

But this site argue cogently that 1,000 is closer to the truth. It also notes:

"There are four houses at Hogwarts, so if there are 1000 students, that means each house has 250 students in it (1000 divided by 4). There are 7 years at Hogwarts. So, if there are 250 students in each house, that means there are approximately 36 students in each year in each house (250 divided by 7 equals 35.7)."

That sounds like a reasonably logical answer to me, even though he uses it to suggest a very much smaller number. And, of course, Ms Rowling has also said that 1,000 is about the right number.

Let us see this through the example of Gryffindor, in the first year, 5 boys, Harry, Ron, Neville, Dean and Seamus are allotted Gryffindor and in my estimation three girls; Hermione, Parvati and Lavender are put in Gryffindor. So if 8 or 10 students are given to each of the four houses, then apparently, there have to be 40 students approximately in every year. Since there are seven years, then doing the math, we can assume that there are around 280 students in Hogwarts! Phew!

That’s not a lot now, is it? Include the professors, Hagrid, ghosts, portraits, poltergeist, Filch, elves, and other magical creatures and of course the pets and then maybe the castle of Hogwarts might get to seem succulent!