How many faces does a heptagonal prism have?

Nine, if the prism has both a base and a top (plus the seven sides). Nine faces is customary for a heptagonal prism (but there are variants ... see below).

If the prism is shaped in the form of a pyramid, then eight. There are seven faces, above the plane, plus its base, making eight total faces. (Hepta- is the Greek root Word for the number seven.)