Full Idea
The definition of 'number' as a multitude of units is appropriate only for whole numbers.
Gist of Idea
Only whole numbers are multitudes of units
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.15)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.156
A Reaction
One can also define rational numbers by making use of units, but the strategy breaks down with irrational numbers like root-2 and pi. I still say the concept of a unit is the basis of numbers. Without whole numbers, we wouldn't call the real 'numbers'.