Ok, J.K Rowling gave a live interview on Scholastic.com where she said Hogwarts had about a 1000 students. To be honest that seems weird because that means there are a lot of students that never get mentioned at all.For example in Harry’s dorm room there are five boys, Neville, Harry, Ron, Seamus and Dean and no other boys in their year and house are ever mentioned.As far as the girls go there’s Pavarti, Lavender and Hermione, in other years no more than three students of the same sex and house are ever mentioned
If you went off 5 people of each sex per house per year you would end up with a school of about 280 pupils. Also why would they need to split classes with other houses if there were enough pupils from each house to make a full class? If there are 1000 people split between 7 years and then 4 houses that’s about 35 students. That’s a pretty big class, if they have a double class that’s a crazy 70 students to one teacher.
Anyway there are not very many wizards out there. My senior school had about 1000 pupils for a town of 10,000 people and a catchment area of maybe 5000 more? So there are perhaps 15,000 wizards in Britain (and presumably Ireland seeing as Seamus goes to Hogwarts) . So why is the Quidditch World Cup Stadium big enough for 100,000 people? I know there are a lot of foreign wizards there but that’s six times the wizarding population of the entire country. That’s really really big. Like if Wembley sat 36 million. Maybe there are a few House Elves and Veela around too but Goblins and Centaurs don’t seem that into Quidditch so magical creatures don’t make up the deficit.
There’s also the problem of why there are so few Wizards. Magic doesn’t seem to work that well in terms of genetics because of the whole muggle born phenomenon. You don’t need a Wizard relative to be a Wizard (Although Rowling has now said that muggle borns will have a wizard ancestor somewhere, but even so if you go back far enough everyone is related to everyone so chances are everyone would have one wizarding ancestor somewhere). However, simply in terms of numbers, squibs are rare, muggle borns seem less rare, people with one wizarding parent seem to end up as wizards. Therefore the amount of Wizards ought to be growing pretty fast, if it was a genetic thing it would be a dominant allele. Oh and persecution of Witches and Wizards controlling the population doesn’t work because if you’ll remember in the Prisoner of Azkaban Harry has to do History of Magic homework on the pointlessness of witch hunts.
As far as Hogwarts goes we’re just going to have to assume there are a lot of students that just never get a mention. There’s also a neat little theory floating around that Voldemort is screwing the numbers. Like Harry’s year would be very small because at the time of his birth most wizarding families were more worried about surviving than procreating.
The size of the stadium could be explained by the fact that it’s much easier for foreign Wizards to travel for a sports tournaments than it would be for muggles so they can rely on a huge number of foreign wizards to fill the stadium and even if there were only 15,000 Wizards in the UK there would be more than a million globally. And a small amount of wizards are homeschooled so that might mean the population is somewhat larger. As for the small wizarding population in general, well throughout wizarding history wizarding families have been reluctant to marry muggles or train muggle borns and it’s always possible that there are a large number of half bloods who end up as muggles but never get a mention. Also wizards might tend to have smaller families than muggles because they have access to magical birth control. There also.seem to be a great many wizards who never have families e.g. Approximately all the teachers at Hogwarts. And we’ll throw in Voldemort’s rein of terror for some added population control.
Extra details, J.K Rowling said that she thought there would be about 3000 wizards in Britain. This make the World Cup stadium even more ridiculous and strengthens the argument for a small Hogwarts. There is no way a third of the wizards are at school. Numbers are not your strength JK. Sort it out.