The arrival roster at Deep Space 9 mentioning Deneb XX
The location of Deneb in "The Explored Galaxy" wall chart
Deneb XX was the twentieth planet in the Deneb planetary star system. This system was located in the Alpha Quadrant. This was one of four inhabited planets in this system; the others were Deneb II, Deneb IV, and Deneb V.
Deneb XX was visited by the GHD Per'ot in 2370 prior to the ship's arrival at Deep Space 9 on stardate 47566.8. (TOS: "Where No Man Has Gone Before", "I, Mudd", "Wolf in the Fold"; Star Trek VI: The Undiscovered Country, production art; DS9: "Whispers")
Background
Deneb XX was never mentioned or shown; it was simply listed in an okudagram graphic of an arrival roster on a viewscreen. Deneb XX had the highest ordinal of the canon planets.
"The Explored Galaxy" was a map of charted space in the Alpha Quadrant. Deneb was located between the Tholian Assembly and Romulus. Both locations were identified in DS9: "The Search, Part II", "Call to Arms" as being located in this quadrant. This star chart was also seen in several Star Trek: The Next Generation and Star Trek: Deep Space Nine episodes set in the 24th century, from the year 2364 to 2370. These were: TNG: "Conspiracy", "The Measure Of A Man", "The Emissary", "The Mind's Eye", "The Game" and DS9: "In the Hands of the Prophets", "Cardassians".
Star Trek: Star Charts
The author and illustrator, Geoffrey Mandel, believed that there were two systems that shared the same name. The two systems were Deneb (Deneb Kaitos) and the "true" Deneb (Alpha Cygni). Deneb Kaitos was a system with five planets and a failed protostar. The number of planets in the Alpha Cygni system wasn't stated in the book, so it might be probable that Deneb XX was located in this system.
Located about 3,230 light years from the Sol system, the Alpha Cygni system was located in the Alpha Quadrant. This system was a single star system. Primary was a Class A star with a magnitude of -3, which was a thousand times brighter than Sol. (pgs. 12-13, 41)