Maximum Number of N.O.C's for applying higher posts in RRB ,Other central govt jobs and to state govt jobs

 please aware me to know the rules against issue of  N.O.C to apply higher grade jobs to RRB,Central Govt Jobs and State Govnt jobs. Is there any rule to issue specific number of N.O.C and related issues .please enclose specific location it was incorporated in Establishment rules or attach the railway board letters against this issue please.    

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