Full Idea
Frege's definition is that the number N F(x) of F's, where F is a concept, is the extension of the second level concept 'is equipollent with F'.
Clarification
'Equipollent' means they map one-to-one onto each other
Gist of Idea
The number of F's is the extension of the second level concept 'is equipollent with F'
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by William W. Tait - Frege versus Cantor and Dedekind III
Book Reference
'Philosophy of Mathematics: anthology', ed/tr. Jacquette,Dale [Blackwell 2002], p.43
A Reaction
In trying to pin Frege down precisely, we must remember that an extension can be a collection of sets, as well as a collection of concrete particulars.