First question : is there a limit of questions with LimeSurvey (500 questions and even more ... (Several questionnares has to be filled in) I should create 1 survey, with lots of them... Or is there a limit for the number of questions? (Student Version)

Second question : Is it possible to create 1 survey, for both partents and students? Without being able reading each others answers? (student version)

There is no such thing as a 'student version'. There is just ONE version, it is free, no matter if you are a student or a big coporation.

Now there is also no 'real' limit regarding questions or number of questionnaires in Limesurvey. I say 'real' limit, because there are of course limits, but they do rather depend on general limits (database, underlying server) than on Limesurvey.

Depending on the type of questions, the database engine could cause you to run into limits. To be honest, I think it will be difficult to find respondents that will respond 500 questions. But maybe you can explain a little bit more in detail, because honestly, your description is rather confusing.

About the other problem: I am not quite sure what you are referring to? Respondents usually are not allowed to see the results. Results (if you don't make them available as public results) are usually only seen by users that are logged in to the admin interface of Limesurvey.