Scott is correct. The term "ultrasonic" is not very specific. It refers to a frequency that is higher than a person can hear. Typically, a person with good hearing can hear up to around 20kHz (20,000 Hz).
So, "ultrasonic" means frequencies above 20,000 Hz.
There is no upper limit.
In practical terms, the upper limit is determined by the medium the waves are traveling through. (In the air, if my memory is correct, the upper bound is something like 10GHz. Anything higher than this, and the air can't vibrate fast enough to propagate the wave)