The IR is scored out of 8 so a full score would be 8/8. You can see a copy of an official score report here
As for the number of questions you get correct - it's a big complicated to explain. You might have between 2-4 experimental questions out of the 12 IR questions. This means that in your set of 12 questions, only 8-10 of them might count. So out of these 8-10 you probably have to get around 5 questions correct to get the IR score of 4-5 that you are looking for. How you score on those 2-4 experimental questions - whether all correct or all wrong - will have no bearing on your 8/8 IR score.
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