How many different nitrogen bases are there (including those found in DNA and RNA)?

You have the five standard bases. However, nitrogenous bases in DNA and RNA are pyrimidines or purines. These are cles of organic molecules that have many other members. For example, some non-standard bases found in RNA are pseudouridine, inosine, hypoxanthine. The number of different possible bases is quite large as changing atoms and functional groups on the rings will create chemically distinct compounds. The choice between 20 and 26 is in your lecture notes.