A maximum of 4 attempts is permitted to every candidate and 7 to those belonging to Other Backward Classes. There is no restriction on the number of attempts in the case of SC/ST candidates. All this provided you are still under the age limit. Also it is wiser to be mentally ready for several attempts as cracking the Civil Services Exams is a tough nut to crack the first time around; and if you do qualify you may want to attempt again to improve your ranking and therefore your service allotment.