Deneb XX has only been mentioned in the arrival roster okudagram. Deneb XX has the highest ordinal of the canon planets. In "The Explored Galaxy" star chart, Deneb was located between the Tholian Assembly and Romulus. Both were identified in DS9: "Call to Arms" as being located in the Alpha Quadrant.
According to Star Trek: Star Charts (pgs. 12-13, 41), there were two systems that shared the name Deneb. Deneb Kaitos system only had five planets, suggesting Deneb XX was located in the Alpha Cygni system.