Bit of a subjective question, so my answer might be a bit vague - depends on the subjects you're doing and what you define as "failing". Obviously the top 4 subjects used to calculate your ATAR will contribute a lot more to your ATAR than the 5th and/or 6th increment subjects. For example, when I did year 12, me scoring 1 higher in my 4th subject would have made a greater contribution to my ATAR than me scoring 5 higher in my 5th or 6th subject.
Being a few years out of VCE I hope that I can help reassure you - the absolute best thing to do is concentrate on studying well and smashing your exams. That's the only way you'll be able to control how your ATAR is calculated - there's no point in stressing about how it's calculated or hypothetical situations where you get x score for this subject and y score for that subject, because you don't have control over the maths/spell casting VCAA do when they're calculating the numbers - you just have control over what you write on the exam paper on the day

All the best!