I would say about more than 6
You need to pass with 25/30. If you get 6 wrong you fail.
30 questions. 6 wrong=fail
All of them - you'd fail, but the question did not say anything about passing the test!
You could fail seven questions, and still meet the minimum mark required.
You can get up to 150 questions wrong.
http://dmvanswers.com/questions/297/How-many-questions-do-I-have-to-get-right-to-pass-drivers-permit-test
To pass the test you need 70% of the questions right.
5
You can get them all wrong. You won't pass the test, but that is not a requirement of the question.
20
11
37 questions wrong gives you a 70.2% correct.