Madam Hooch puts out 20 brooms for the first flying lesson for the Gryffindor’s and Slytherin’s in Harry’s first year. We can infer from this that there are 20 students total in Slytherin and Gryffindor House.
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We know who 15 of those students are from the sorting that year. The Gryffindors: Harry, Ron, Dean, Seamus, Neville, Hermione, Lavender and Parvati. The Slytherins: Draco, Crabbe, Goyle, Zabini, Nott, Millicent, and Pansy. That leaves 5 unknown students.
The OCD in me wants the classes to be split evenly, 10 total in Gryffindor, and 10 total in Slytherin; and then also even within each house when it comes to boys and girls. If this was this case, that would mean there are 2 unknown Gryffindor girls, and 3 unknown Slytherin girls.
There are 2 female names during the sorting ceremony that we don't get to hear what house they were sorted into: Moon, and Perks, Sally Anne. Perhaps they are 2 of the unknown students of Gryffindor or Slytherin?