Full Idea
Treating numbers adjectivally is, in effect, treating the numbers as quantifiers. Frege observes that we can always parse out any apparently adjectival use of a number word in terms of substantival use.
Gist of Idea
Treating numbers adjectivally is treating them as quantifiers
Source
Crispin Wright (Frege's Concept of Numbers as Objects [1983], 1.iii)
Book Reference
Wright,Crispin: 'Frege's Conception of Numbers' [Scots Philosophical Monographs 1983], p.10
A Reaction
The immediate response to this is that any substantival use can equally be expressed adjectivally. If you say 'the number of moons of Jupiter is four', I can reply 'oh, you mean Jupiter has four moons'.