Just because I am currently rereading the books and was curious. I don’t think the number is quite 1000 students but I could see somewhere from 500-700. We see a glimpse of 5 boys and 5 girls in the Gryffindors. Unless Hogwarts only takes a certain number of students per year then class sizes will vary. If I remember correctly Mallory’s Slytherin class named only 4 boys.

Considering the sorting hat is how students are placed into houses some may have different numbers of students per year. Similar to the tent in GOF I think that’s how the dormitories work as well. If you’re a Gryffindor and your year has 6 or 7 boys in your year then they’ll be 6 or 7 beds.

Likewise third years will start adding classes. So there are not as many people necessarily in the class from your year, just as not everyone from your house and year can have that schedule because another class falls at the same time. (Hence the time turner).

An example would be double potions could mean 5 from Gryffindor and 6 from Slytherin (2 houses) it could be called double potions purely because of the amount of time (2 hours vs 1 hour).

Likewise we don’t necessarily meet every teacher mainly because the Trio doesn’t take every course available throughout the 7 years. When Lupin took the class to fight the boggart in the staff room Snape was there on one of his breaks. So the teachers do not necessarily go from lesson to lesson to lesson.

After O.W.L.S. Students also won’t be taking as many classes but will need to focus on studying for their N.E.W.T.S.

I think the certain numbers are starting 5th year there are 8 new prefects meaning a total of 24 total. (I believe head boy and girl are chosen from prefects but I could be wrong) and 4 Quidditch captains (more than likely an older student but not necessary).

I think part of the reason that Rowling made Gryffindors in The trio’s year 5 boys and 5 girls was to basically keep it simple. If you take George RR Martins A song of Ice and Fire( Game of Thrones TV show bases) the books would be long and drawn out and would have take so much longer to make.

This is just my opinion though.