Galaxy

According to the Tenth Doctor, universes such as N-Space contained millions, (TV: Planet of the Ood) potentially trillions of galaxies while constantly expanding. (PROSE: The Last Dodo) The Tiberian spiral galaxy, for one, contained a million star systems, a hundred million planets, and a billion trillion people when it was destroyed. (TV: Nightmare in Silver)

Galaxies

Most of the Doctor's adventures took place in the galaxy known to the Time Lords as Mutter's Spiral, (TV: The Deadly Assassin) although he was also known to have visited other galaxies such as the Andromeda Galaxy and the Isop Galaxy. (PROSE: Alien Bodies, TV: Castrovalva, The Web Planet)

The Daleks recreated the Seriphia Galaxy with the Apocalypse Element and used it as their main base of operations during their invasions of Mutter's Spiral. (AUDIO: The Apocalypse Element, Dalek Empire)

The planet Drahva, home of the hierarchical, feminine-appearing Drahvins, was located in Galaxy 4. (TV: Galaxy 4)

Urlic was the last planet of the Catorkian Galaxy by 2050. (TV: Lost Library of Ukko)

In the year 4000, the Daleks held a meeting with representatives from seven Outer Galaxies to plot the invasion and overthrow of human-occupied space. By that time, they had already taken over at least seventy planets in the Ninth Galactic System. (TV: Mission to the Unknown)

The Tiberian spiral galaxy was a galaxy sacrificed by the human empire to destroy all Cybermen, bringing the Cyber-Wars to an end. The human emperor, Ludens Nimrod Kendrick Cord Longstaff XLI, ruled a thousand galaxies. (TV: Nightmare in Silver)

Whilst travelling with the Tenth Doctor, Donna Noble witnessed the destruction of the Mestophelix Galaxy. (COMIC: The Widow's Curse)

The Five Galaxies were a group of galaxies which included Mutter's Spiral. (PROSE: Borrowed Time, TV: Delta and the Bannermen)

The Seven Galaxies were a collection of galaxies near and at the centre of the universe. (TV: Twice Upon a Time) The Kaleds believed that Skaro, which existed in the Seventh Galaxy, (PROSE: The Infinity Doctors) was the only planet capable of supporting life in all of the Seven Galaxies. (TV: Genesis of the Daleks)

The Ten Galaxies were a group of galaxies ruled by the Dominators and their Quark servants, calling themselves the "Masters of the Ten Galaxies," (TV: The Dominators) although the claim went unchallenged (PROSE: Mutually Assured Domination) and William Bishop later wondered if it was simply propaganda. (PROSE: The Two Brigadiers) Among the galaxies were Phi-Ralix, the Dominators' home galaxy, the Mutter's Spiral (PROSE: Mutually Assured Domination) and, in the far future, the galaxy QSO 0046. (PROSE: Timewyrm: Apocalypse)

The Twelve Galaxies were a group of galaxies. (TV: Dragonfire, The Ghost Monument)

Galaxies designated as junk galaxies were home to junk planets. (TV: The Tsuranga Conundrum)

Unidentified galaxies

Eugene Jones supposedly "worked out" which galaxy the Dogon Sixth Eye hailed from. (TV: Random Shoes)

According to Jack Harkness, the space whale known as "Cash Cow" hailed from a galaxy beyond Mutter's Spiral. (TV: Meat)

Sarah Jane Smith observed that Erasmus Darkening hailed from a galaxy other than Mutter's Spiral. (TV: The Eternity Trap)

The Bank of Karabraxos was promoted as the greatest and most secure bank in "the galaxy". (TV: Time Heist)

Fate

In N-Space, the galaxies had collapsed by the end of the universe in the year 100,000,000,000,000. The Tenth Doctor told Professor Yana that, had he been in a different time, he would have been revered throughout the galaxies. (TV: Utopia)

Other references

In 1958, Lord Azlok boasted that the Viperox had laid waste to entire galaxies. (TV: Dreamland)

The Eleventh Doctor noted that Saborian smelling salts were outlawed "in seven galaxies", (TV: The Power of Three) while magno-grabs were outlawed in "most galaxies". (TV: Journey to the Centre of the TARDIS)

The Moment was known as the "Galaxy Eater". (TV: The Day of the Doctor)

Davros reflected that galaxies had burned in the time that he and the Doctor knew each other. (TV: The Witch's Familiar)