Full Idea
'Ten men' is grammatically the same form as 'white men', so that 10 might be thought to be an adjective qualifying 'men'.
Gist of Idea
Maybe numbers are adjectives, since 'ten men' grammatically resembles 'white men'
Source
Bertrand Russell (Introduction to Mathematical Philosophy [1919], XVIII)
Book Reference
Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.205
A Reaction
The immediate problem, as Frege spotted, is that such expressions can be rephrased to remove the adjective (by saying 'the number of men is ten').