We very often see individuals worried about getting an appointment. It’s something very trivial. Take one thing at a time. First prepare for GMAT, small issues like these would take care of themselves and should not be given undue consideration.
There are two strategies that can be followed:
Take an appointment well in advance. Say 45-90 days before your GMAT and use the deadline as a motivation to prepare for GMAT.
Keep on preparing for GMAT and when you think that you have reached the peak, take an appointment of just 2-3 days later. Since one needs a gap of 7 days to re-schedule an appointment, many slots are cancelled just 7 days before. Hence, if you look for an appointment just 2-3 days ahead, you will almost always find one. If your luck is really against you, look for a less popular testing centre in your vicinity and you will definitely find a slot there. E.g. If you stay in north India and are not getting a slot in Delhi, look for Allahabad.