A granum is a stack of thylakoids, so it is made up of multiple thylakoid discs.
You can see in this diagram that the chloroplast has many discs inside it. These are called thylakoids and they are the site of the light dependent reactions of photosynthesis.
A granum is a single stack of these discs. If you are referring to more than one granum, you would use the term "grana."