Full Idea
Frege's answer is that the concept of number is fundamentally bound up with the notion of one-one correspondence.
Gist of Idea
Frege thinks number is fundamentally bound up with one-one correspondence
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Richard G. Heck - Cardinality, Counting and Equinumerosity 1
Book Reference
-: 'Notre Dame Journal of Formal Logic' [-], p.190
A Reaction
Birds seem to find a mate with virtually no concept of number. I'm beginning to think that the essence of numbers is that they are both ordinals and cardinals. Frege, of course, thinks identity is basic to metaphysics.