Black patches of skin in your mouth can be very serious or something benign. The serious one to be concerned about is malignant melanoma, which is a very aggressive cancer. The most common benign dark patch is called an amalgam tattoo. Amalgam tattoos are usually near a silver filling and are caused by a portion of the amalgam/silver filling being lodged in the tissue and over the years discoloring it. It would be very odd to have more than one in your mouth and they tend to be rather small. Obviously the previous post about stopping tobacco use is important, but the most important thing to do as soon a possible is to have that tissue evaluated by someone other than a general dentist or general doctor. You need to see someone who specializes in these kinds of things. If there is a university near you that has a dental school, all of them have an oral medicine specialist on staff. This would be right in that person's wheelhouse. The other suggestions of seeing an ENT who is a head and neck surgeon is also a good choice as they have seen plenty of melanomas.