Well the maximum you can be asked for is 3 if they're feeling nasty (not commonly known fact, but it's true!). They've never done it yet, but be warned: every year they don't makes it more likely they will the following year.
However, sometimes your related text won't fit the question. It happened to me. Luckily, I was prepared. I had 6 supplementary texts for Change (now Journey, but I did the HSC last year), 6 for Section C (Telling the Truth), and 10 for Extension I (Crime Fiction- if anyone's doing that here!). I would recommend at least 4- I'm a fast reader and loved the subject so I did more than most people.