###Different versions of Superman at different times in his 75 year history touted him as the Last Son of Krypton. And other versions of the character would include literally millions of survivors he was not initially aware of being held captive in the Bottle City of Kandor.

The answer to this question depends primarily on editorial fiat. Some periods in Superman's history they wanted him to truly be the Last Son of Krypton. John Byrne's Man of Steel run was such a period.

 - Early versions of Superman (especially the Golden Age Superman) had him being the only survivor of his lost home planet Krypton. 

 - But by the Silver Age of comics no one counted on the popularity of this strange immigrant from the stars. Eventually, he would get a cousin (Kara Zor-El) who survived Krypton as well but would not arrive on Earth until after he had reached adulthood. He welcomed her, trained her in the use of her metahuman abilities and even allowed her to wear his symbol and colors.

 - Over time, other survivors would be less welcome. When Kal-El learned about the Phantom Zone projector and its White Zone, where prisoners of the former Krypton placed the worst of their criminal element. These terrifying villains would escape the Phantom Zone and periodically wreck havoc. In some realities, they would escape the Phantom Zone and kill every living thing on Earth.

During the Silver Age and thereafter, different continuities will eventually have Superman either save, find or defeat Brainiac who managed to create a device capable of shrinking an entire city and all of its inhabitants small enough to fit inside a bottle. This becomes one of the most famed artifacts in the Superman Mythos: **The Bottle City of Kandor.** 

 - The shrinkray technology was used on Krypton by the evil Coluian, Brainiac (later rewrites will have Brainiac become a machine intelligence spawned on Krypton). Brainiac inadvertently was responsible for the entire vast city of Kandor and its 6 million inhabitants being able to survive unscathed. 

 - Held prisoner by Brainiac, the Kandorians were kept under a red-sun emulator, preventing the Kandorians from developing any superhuman powers unless released from the city. In the last iteration of the DC Universe, the Kandorians eventually leave and establish New Krypton, a colony planet of Kryptonians. Let's just say this doesn't end well for the Kandorians.

 - During the Silver Age Kal-El would even acquire a collection of Kryptonian superpets (who looked exactly like their Terran equivalents...) complete with intelligence and superhuman powers similar to Superman's. This has not been repeated in later eras except for the creation of Krypto, the Last Dog of Krypton.

 - Kryptonian artifacts would also play a part in the Superman mythos. Devices such as the Eradicator would clone Superman and try to impose a more rigid Kryptonian dogma on Earth during the Death of Superman Saga. 

Superman's history is very long and has been through a number of rewrites, continuity changes, complete universal reboots and contingent parallel realities. Sometimes he is truly the Last Son of Krypton (See: Red Son, and The Nail) and in some of them he is the head of an entire pantheon of godlike beings. (See: DC: One Million)