You can take the Diagnostic Test as many times as you would like. In order to retake the test, you will first need to "Submit Final Answers" from your previous attempt. Then, you can start a new attempt.
The Diagnostic Test is NOT the same as the Practice Test. The Practice test is only available at the end of the study guide and you are allowed 3 attempts. The Diagnostic test is used to help you gauge your strong and weak areas so you can tailor your studying. You have an unlimited number of Diagnostic Tests.