Failing one sac won't be much detriment to your final mark
Failing many sacs will become the bane of your existence.
+1 (I don't know about the last part though haha).
SACs make up a certain percentage of your final mark. You should be able to find that out without too much trouble. I didn't do VCE English (but instead VCE English Language) but it may be something like 50%. That would be divided into 25% each unit. Then you would calculate how large your SAC was relative to that 25%. Let's say 20% of Unit 3 which means that it would be 5% of your final score. So take the relative percentages etc. into account.
If you got eg. 4/10 then that doesn't mean that you will have destroyed your study score, because of factors such as if everyone else did badly, then it probably won't mean too much to your score. There many more factors into play and possible variables you could weight against this matter which can determine how well you go.