попробуйте
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
//Set tag for a textfield
if(textField.tag==10)
{
NSUInteger newLength = [textField.text length] + [string length] - range.length;
return (newLength > 20) ? NO : YES;
}
}