How many cities made up the league of Hellenized cities known as the Decapolis?
Answer: As many as 19
The Decapolis (Greek for “ten cities”) was a loosely aligned group of heavily Hellenized, Greek-speaking cities of Greco-Roman Palestine, located primarily in the northern Jordan River Valley and the fertile highlands of northern Transjordan. The only member city west of the Jordan was Beth-Shean (Scythopolis).
Tracing their roots to the colonial conquests of Alexander the Great and his Macedonian successors during the late fourth and third centuries B.C.E., the Decapolis cities became centers of Greek culture, architecture and learning. The cities were laid out and built to the highest Hellenistic standards, with paved, colonnaded streets and lavishly decorated theaters, temples, baths, marketplaces and hippodromes. Their impressive remains include some of the major archaeological sites of present-day Israel and Jordan, including Beth-Shean, Hippos/Sussita, Amman (Philadelphia) and Jerash (Gerasa).
For the ancient writers who used the term Decapolis, however, there seems to have been more than a little confusion about how many—and which—cities actually made up the “ten cities.” The earliest list, compiled by Pliny the Elder around 77 C.E., contains ten cities (Damascus, Philadelphia, Raphana, Scythopolis, Gadara, Hippos, Dion, Pella, Gerasa and Canatha), but he also notes that “not all writers keep to the same list.” This seems to have been the case for the second-century C.E. geographer Ptolemy, who adds nine new cities to the list (Heliopolis, Abila, Saana, Hina, Abila Lysanias, Capitolias, Edrei, Gadora and Simulis), while dropping one (Raphana) that Pliny had included—for a total of 18. Similarly, Stephen of Byzantium, writing in the sixth century, lists 14 cities as part of the ancient Decapolis.