Try this below code, it works fine:-
NSMutableAttributedString *muAtrStr = [[NSMutableAttributedString alloc]initWithString:@"2"];
NSAttributedString *atrStr = [[NSAttributedString alloc]initWithString:@"\ndays" attributes:@{NSFontAttributeName : [UIFont fontWithName:@"HelveticaNeue-Bold" size:8]}];
[muAtrStr appendAttributedString:atrStr];
self.lbl.numberOfLines = 0;
[self.lbl setAttributedText:muAtrStr];
Note:- Also put numberOfLines to 0 for allowing any number of lines