Consider: “I have been able to survive with fewer calories and less advice”.
Fewer and less are adjectives which refer to something diminished and are broadly synonymous, but they are not to be used interchangeably. To determine which is proper, we must look to the noun they modify; in the sentence above, “calories” is a count noun and “advice” is a non-count noun.
A count noun (which is modified by fewer) normally appears in the plural and a non-count noun (which is modified by less) generally appears in the singular. We say “less tea” but “fewer tea bags”. (That is, if we are referring to “tea” generically; specifically, if we were speaking of types of tea such as Darjeeling, orange peako, and Earl Gray, teas would be plural and therefore a count noun.)
It can be a bit more challenging with the “ic” nouns. We would not use fewer with “politics”, “italics”, or “linguistics”, which are all non-count nouns (and would be modified by less) even though they appear to be in the plural. On the other hand “antics” and “critics” are count nouns (they are in the plural) and would be modified by fewer. A helpful tool: With the “ic” nouns, if it makes no sense to use a number with them, as in “two politics” or “four linguistics”, they are non-count nouns.
In summary, when describing a concept or noun which is singular, not numerical, less is the adjective; but if the concept/noun is plural and numerical, use fewer.
We cannot close without commenting on Mies van der Rohe’s famous, oxymoronic phrase: “Less is more” (referring to the architectural and design complexities of the baroque and Victorian eras, contrasted with the stark simplicity of the Bauhaus school of which van der Rohe was a charter member). And then, the playful retort of Robert Venturi: “Less is a bore”. In this era of post-modernism as detail in design has returned, and the stark unadorned buildings of the mid-20th Century have fallen out of favor, Venturi rules.
– Ken Butera