How common is it to have one allele in the normal range and one allele in the mutable range, for huntingtons?

Not rare for some: In families with huntington's, only one allele need be abnormal for the disease to manifest, that is what it means that it is an "autosomal dominant" disease. If you have been tested, you should have access to a genetic counselor, and that is with whom you should discuss your results. This is an important issue for you, and you should get the best possible full explanations, not this website's.