可以使用substringToIndex。

NSString *mystring = @"This is a test message having more than 20 characters";
NSString *newString = [mystring substringToIndex:20];
NSLog(@"%@", newString);

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NSString *str = @"123456789012345678901234";
NSString *subStr = [str substringWithRange:NSMakeRange(0, 20)];

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NSString *string = @"I am having a simple question.I am having NSString and i want a substring of it that contains first 20 ";
NSString *first20CharString = [string substringToIndex:20];

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试试这个：

NSString *string = @"This is for testing substring from string";
    NSInteger intIndex = 20;
    NSLog(@"%@", [string substringToIndex:intIndex]);

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检查以下内容：

NSString *strTmp = @"This is test string to check substring.";
NSInteger intIdx = 20;
NSLog(@"%@", [strTmp substringToIndex:intIdx]);

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这对你会有帮助的。
干杯！