You can sit your first exam/s whenever you feel ready to, its entirely up to you. Once you take your first sitting then the clock starts ticking(18 months from then to finish the remaining exams. You are allowed a total of six sittings therefore you cannot do one exam at a time. Most people try and do them over two or three sittings, leaving spare sittings for any resits. I did mine over 3 sittings(4 subjects, 4 subjects and then the remaining 6.)