There is no limit to how many times you can use a morning after pill, even in the same month. There is no evidence of longer term harm from the use of morning after pills.
However, if you find yourself using morning after pills regularly, you should consider a reliable regular contraceptive for the following reasons:
1. Regular contraceptives are more effective at preventing pregnancy than morning after pills. Most regular contraceptives are over 95% effective at preventing pregnancy, whereas morning after pill are around 75% effective.
2. Repeated purchase of morning after pills can be expensive, and is more costly than most regular contraceptives.