This is question that has no definate answer.
By her writing, Rowling hints that there would be 5 boys and 5 girls per house per year thus making a total of 40 students per year. This number is very low and does not match with the fact that the school is said to be full of students.
However, since finishing the books, Rowling stated that was merely what people had assumed and that there may be other characters that she had not written about. This would therefore push the total to a greater amount.