"How many students are there?"
Translation:Скільки там студентів?
I haven't understood yet when we have to use студентів instead of студенти. If someone could explain it to me please
Студентів is the Genitive form of студенти. Genitive is always used with the question word скільки.
Think of it as "How many of students are there?" :) A sort of literal translation of the Ukrainian grammar pattern, might help remember it.