Less is commonly confused with fewer. They are comparative forms that are used differently based on the noun they are applied to.
Less is used with uncountable nouns such as water, milk, and chemicals and so on. You cannot count quantities of these nouns. For instance, “He put less water in the glass than was instructed.”
Fewer on the other hand is used with countable nouns such as stones, mangoes, desks, tables and so on. For instance, “There were fewer seats than last Monday.”