If, at some point in the exam, you have 6 partial solutions (or $n$ (almost) full solutions and $6-n$ partial solutions where $n=1,2,...,5$), the best thing to do would be to work on your partial solutions starting with the ones you think you're most likely to make progress on. I would strongly advise against doing more than 6 questions unless you think you have 6 perfect solutions already and a lot of time left (even then, it'd be probably be better to check your existing solutions.) The only exception to this could be perhaps where you have attempted 6 but 1 or more of these were a "false start" where you wrote down a few lines but made no significant progress and couldn't see how to proceed. Good luck!