Our 10 week old shih tzu sleeps approx. 18-20 hrs per day.
Is this normal?

It is usual for puppies to sleep a good deal and pups do differ, more importaint how is it when it is awake, is it playful and happy and is it eating OK? If so don't worry in a few weeks you will be wishing it would stop rushing about, but if it is sleepy all the time then I think a visit to a vet is called for.