So basically I have a total of 9 questions in a group and what I want is that during the survey only 6 questions from this particular group are randomly asked.
I remember once doing it with random() function on SurveyCTO but I can't recall how I did that and whether it is possible to do it on ODK.
There is a functionality to randomize the order of the options, see Randomizing choice order.
I'm not sure about randomly showing a certain number of questions from a group of questions. You might try searching past posts.
You could also check the links and add to the conversation here: