Fair Enough

My point isn't that there aren't classess they take with other houses, but rather that once its established which houses share classess it didn't really seem to change until N.E.W.T classes(which seem to be a mixture of all houses). And it was fairly obvious in PoA that the Gryffindors take DADA alone.

 

Personally I think the 20 000-30 000 number is also pretty much what I'd expect, but I divide then up a bit differently.

First of all, life expectancy. My headcanon says that long life is an innate magical "affinity" like others (parseltongue, metamorphmagus, wand affinities), and that there's a huge variance to how long a "natural" lifespan of a wizard can be. Some live comfortably up to 300 years, others "burn out" at the age of 60. Maybe the Blacks at some point of their history did some type of a ritual that reduced their life expectancy but made them more likely to be powerful wizards, explaining why the Black family tree is full of people who died young. The overall average age of natural death is somewhere around 120, which makes the lack of elderly people less jarring than a generation higher life expectancy.

Dividing by that you have the size of a cohort of around 200 people.

Next part of my headcanon is that Hogwarts has annual intake of around 60 students (with Harry's year being a bit smaller due to the war), and the rest are schooled at home or attend a number of smaller, less prestigious schools, or are in some kind of apprenticeship positions with different craftspeople. Maybe they only learn magic that relates to farming, and none of that fancy and useless matchstick into needle general transfiguration that's taught in Hogwarts. That keeps Hogwarts at elite status as "the best school", and explains why so many in Harry's year are related to someone important in the wizarding world. Additionally in my headcanon Hogwarts also takes in _all_ muggleborn children (around 10-15 each year) free of charge as per original charter that caused Salazar Slytherin to leave. So Hogwarts has massive over-representation of both high-class children an muggleborn.

 

The mention of other schools takes you out of canon and into fanon there, its established that Hogwarts is the only school in the UK.

 

They have Potions and CoMC with Slytherins and Herbology and Muggle Studies with Hufflepuff. Doesn't really make sense to have some classes shared with other houses and some alone.

Also, in CoS, Malfoy made fun of Harry giving out signed photos right outside Lockhart's classroom. He might've had Defence with the Gryffindors.

 

Except that only sharing some classess isn't unusal, and we are often specifically told when they share classess with others. Also again, its pretty clear that DADA is a Gryffindor only class during the Boggart lesson in PoA.

No, Malfoy made fun of Harry in the courtyard after dinner when Colin asked him for the Photo, Lockhart was there but its pretty clear that its not near the Defense Classroom since the Text specified that he and Lockhart entered the Castle and went there together.

 

I'm not specifically disagreeing with you, but I think that if you're teaching a class where people's greatest fear will be exposed to everyone in the room, then restricting the number of people present and so trying to ensure that there won't be consequent bullying as a result of your class could only be described as "good teaching practice".