The Concept questions can be attempted any number of times while each Chapter-wise and Practice Test Series can be attempted only once.

However, you may come back and review your tests any number of times. 

The android app also lets you take these tests and it is also treated on an equal basis. So, a test taken on an app is also considered as an attempt. The results of the test taken on the app will automatically get updated on the server when you have internet connection.