Hello Hirama666
There is no limit to the number of users or students in RosarioSIS.
Still, there is a hard limit of 1000 in lists.
That is, typically if you have more than 1000 students, the list will display at most 1000. This applies to every list, not only students.
One solution (for students) is to create various schools. For example one school for Primary and another for Secondary.