Full Idea
For Frege, in 'Grundgesetze', a number is a class of classes of the same cardinality.
Gist of Idea
A number is a class of classes of the same cardinality
Source
report of Gottlob Frege (Grundgesetze der Arithmetik 2 (Basic Laws) [1903]) by Michael Dummett - Frege Philosophy of Language (2nd ed) Ch.14
Book Reference
Dummett,Michael: 'Frege Philosophy of Language' [Duckworth 1981], p.510