You donīt have to take all the exams at the same time, but from when you take the first exam, you have 18 months to finish them all.
There are also 14 exams, and you are only allowed to attend six exam sittings in total. So you need to take more than two exams per sitting. Plus, imagine that you were on your sixth and last sitting, and you failed one of the exams... you have no more sittings and you will have to take the whole ground course over and start over from the beginning and retake all the exams (and pay all over again too). Most people divide the exams into two or three sittings, and then if they fail an exam, they retake it in one of their existing sitting or add one more sitting at the end. What Iīm trying to say, is that you want to leave yourself a margin of error.