27 was a number.
Through use of a magnetron, the New Dalek Empire transported 27 stolen planets from across time and space to the Medusa Cascade, where they collectively served as an engine for the Daleks' reality bomb. (TV: The Stolen Earth/Journey's End)
When arguing with Clara Oswald over who should hack Ms. Kizlet's software, the Eleventh Doctor, among other factors, listed his 27 brains as to why it should be him. When Clara expressed doubt about this, he admitted to it being an exaggeration. (TV: The Bells of Saint John)