I think that in most languages, when people define sets of data in general and when people create taxonomy for website webpages in particular (webpage categorization), they would mostly name categories in plural but often in singular:

Plural webpage categories for example

• Universes
• Planets
• Countries
• People
• Animals

Singular webpage categories for example

• Philosophy
• Psychology

People could use the naming "Philosophies" or "Psychologies" (the same way they would use "humanities") just alternatively.

What are the linguistic pressures behind categorization?
What linguistic-psychological motive would "point" a person to choose a plural or a singular name?

If I am not wrong, in Mandarin all nouns are singular (unless exceptionally stated otherwise), in Thai all nouns are plural (unless exceptionally stated otherwise) and I am not sure about Japanese.