Hi
The answer is three, IF the contracts follow each other i.e no break. If you are laid off for the winter, then their is no limit to the amount that you can work before the company has to offer you a permanent position, or release you. Its horrible but that is UK employment law for you.
With regard to TCX, I believe their is a list of whose suitable to be retained from the C4's and you progress alsong this list as permies leave. One girl tolk me, their was 10 in front of her and x amount behind. This was because it was her 3rd season and she was on the 1st or 2nd course back in JMC days.