I don't believe there are any hard limits in the code.
However, your
server only has so much memory, and may not be able to cope with submitting big quizzes.
Does it help if you break the quiz up into different pages?
I suspect not many people have tried quizzes with that many qustions, therefore you may be the first people to encounter the performance problems. You will probalby find that they are fixable. For example, at the OU we seem to be the first people to encounter some the problems that are caused by having 150000 registered users. For example the sync_metacourse was taking 20 seconds to execute on some of our courses last week, but was able to rewrite it to take less than 1 second.