Hey! Limiting number of characters is really easy if you have standard keyboard. The link below offers so many options for this prob:
http://stackoverflow.com/questions/7010547/uitextfield-text-change-event

But these options werent fit for me. In my case, I had custom keyboard…
The solution I found was pretty satisfying.
I wrote a function to check max length of a text field. Then I checked this func every time I added character to my textfield.

  @IBAction func addBtnLabel(sender: AnyObject) {
         txtFieldPlateNumber.text =  txtFieldPlateNumber.text?.stringByAppendingString(sender.titleLabel!!.text!)        checkMaxLength(txtFieldPlateNumber as! UITextField, maxLength: 14)
     }

    func checkMaxLength(textField: UITextField!, maxLength: Int) {
         if (textField.text?.utf16.count > maxLength) {
             deleteLastCharacter()
         }
     }

//To delete backwards.
 func deleteLastCharacter(){

        if (txtFieldPlateNumber == 0) {
             // textField is empty
             print("text field is empty")
         }
         else{
             var name: String = txtFieldPlateNumber.text!
             var truncated = String(name.characters.dropLast())                       txtFieldPlateNumber.text = truncated
        }
     }

   Thinking simple, creates simple solutions. 🙂