Comparatives and Superlatives
Much and many are used with count and non-count nouns (e.g., much time but many hours). Their comparative and superlative forms are
much - more - the most
many - more - the most
(A) little and (a) few are also used with count and non-count nouns (e.g., little time but a few hours). Their comparative and superlative forms are
little - less - the least
few - fewer - the fewest
What’s the Problem?
The problem is that language changes over time. There was a time when less and the least were also used as the comparative and superlative forms of few. For whatever reason, some grammarians decided that, because fewer and fewest also existed, it would be preferable to use fewer and fewest to keep the table “neat” (i.e., without alternate forms for few).
People took this suggestion as a steadfast rule. Now, there are still many people who will live by it and “correct” anyone who says something such as “15 items or less (items).” After all, “items” is a countable noun, so it should be “fewer items.”