Full Idea
The number of Fs is the 'successor' of the number of Gs if there is an object which is an F, and the remaining things that are F but not identical to the object are equinumerous with the Gs.
Gist of Idea
The number of Fs is the 'successor' of the Gs if there is a single F that isn't G
Source
Peter Smith (Intro to Gödel's Theorems [2007], 14.1)
Book Reference
Smith,Peter: 'An Introduction to Gödel's Theorems' [CUP 2007], p.120