Full Idea
Individual numbers are objects, as is indicated by the use of the definite article in expressions like 'the number two', and by the impossibility of speaking of ones, twos, etc. in the plural.
Gist of Idea
Numbers are objects, because they can take the definite article, and can't be plurals
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §68 n)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.80
A Reaction
Hm. The beginnings of linguistic philosophy, with all its problems. It is well known that 'for the sake of the children' doesn't make an ontological commitment to 'sakes'. The children might 'enter in threes', but the second half is a good point.