Yes, you are locked out after 5 incorrect prints. At that point, you need to use your passcode to unlock your phone.
That said, even if they guess that you're most likely to use your thumb, there is still a chance you can't unlock the device, especially on pre-6S devices, since the sensor is a lot more finnicky.
That said, even if they guess that you're most likely to use your thumb, there is still a chance you can't unlock the device, especially on pre-6S devices, since the sensor is a lot more finnicky.