SN12-14
SN12, SN13 and SN14 were Starship Prototypes in the SN8+ series that did not complete construction. There are two major factors for their cancellation: 1) Major design changes being accepted for SN15, particularly of the thrust dome design, and 2) the unexpected success of SN8's test flight. The design changes were announced by Elon in a tweet on November 24[1], and it is likely that SN13 and SN14 were cancelled at this time to focus on SN15.
SN12
Serial Number 12 would have been largely similar to SN10 and SN11. Components first appeared at the beginning of October 2020, with the first dome being sleeved by the end of that month. Progress continued as normal through November with the major sections being made ready for stacking, but SN9's leaning incident in the high bay caused knock-on delays in assembly that prevented SN12 from being stacked due the Mid Bay being occupied by SN10 and SN11. Once space was made available, it was initally thought that SN12 would join SN11 in the Mid Bay, but it was later revealed that SN15 was being stacked instead. The cancellation of SN12 was confirmed with the scrapping of the Aft Dome section on January 23.
SN12's nosecone and nose barrel would later be put to use as the nosecone Max-Q test article, inside the MaxQ Nosecone Cage.
SN13
Serial Number 13 would likely have followed SN12's design. The only component positively identified for SN13 was the methane header tank, which was spotted on November 20. It is possible that this header tank found its way into SN15.
A Forward Dome and largely-complete Aft Dome appeared outside the tents in mid November, where they remained for six weeks before being sleeved with 3mm rings to become part of the SN7.2 test tank. It is likely that these would have been used in SN13, had this not been cancelled.
SN14
Serial Number 14 would likely have followed SN12 and SN13's design. Identified parts include a methane downcomer spotted on October 10, the aft skirt spotted on November 27, and a nosecone section spotted on December 14. These sections potentially became part of SN15.
An incomplete thrust dome, missing the reinforcing stringers and weld doublers, appeared outside the tents in late November. This would likely have become part of SN14. This dome remained outside until late January, when a number of holes were cut into the outer dome section as an apparent pathfinding article for Super Heavy's outer engine plumbing.