The Douluo Continent manga's 73 drawings were not the first volume but the 73rd volume. Every comic book would have a corresponding volume number. The 73rd volume corresponded to the seventh volume of the Douluo Continent comic.
The Douluo Continent manga had a total of five volumes, and the seventh chapter corresponded to the fifth volume, which was the "Douluo Continent: The Peerless Tang Sect."
Douluo Continent's comics were currently not officially divided into volumes because comics were usually published according to chapters. However, according to what I know, Douluo Continent Manga's 173rd chapter might appear in the 11th or 12th volume of the standalone version. However, the specific number of copies may vary according to different editors and sales channels.