Apparently the largest integer number that can be processed with NSNumberFormatter is long long, which is 9223372036854775807. Anything beyond that will lose precision and not come out as you put it in.
Instead use NSDecimalNumber, a concrete subclass of NSNumber. And it can even parse strings itself:
NSDecimalNumber *dn=[[NSDecimalNumber alloc]initWithString:@"12790162104953153719"];
NSLog(@"dn: %@",dn);
NSDecimalNumber can handle up to 38 digit long decimal numbers before it loses precision.