Hi Laura,
You can not make sure they answered 10 questions in either a Lesson nor even in a Quiz.
It really has to do with scoring (see Lesson settings). In your example, if the student answers 8 questions and leaves the lesson, the maximum score they could receive is 8/10. If the student answered 15 questions, the maximum score would be 10. (Actually, I am not sure what happens if the students answers 15 questions and answers 9 of the correctly. Just that the max score will not be higher than 10.)
You can tell them the scoring system at the start of the Lesson and let them make their choices. This point is made in Moodle Docs under the min question settings.
Chris