Star Trek Expanded Universe
You may also be looking for the Thirteenth Fleet of Star Trek: Liberty and Star Trek: The Adventures of Argus.

The Thirteenth Fleet was a numbered Starfleet fleet extant in the 23rd and 24th centuries.

Rear Admiral Gregory Young commanded USS Excalibur -- at the time, deployed to the Thirteenth Fleet -- in 2286. (Starfleet Communiqué 20)

In the opening phases of the Dominion War, the Thirteenth was held in reserve, but later deployed to bolster defenses along the Vulcan front. In early 2374, the Thirteenth and the Thirty-First Fleet participated in the First Battle of Vulcanis. For the remainder of the war, the Thirteenth remained on the Vulcan front. (LUG RPG: The Dominion War Sourcebook: The Fires of Armageddon)