How common are soft tissue cancers in the head and neck?

Common. Soft tissue tumors include lipomas, which are one of the most common tumors in soft tissue. Therefore, in the head and neck, these are quite common. If you are talking about sarcomas (malignant soft tissue tumors), they are much less common. The location is important: neck versus larynx (voice box), versus oral cavity or nasal cavity: location is important in determining outcome.
Clarify. If by soft tissue cancers, you mean sarcomas - overall incidence of sarcomas is around 11, 000 per year, of which 10% of those tend to arise in the head and neck region.