Post History

In a language like English, the distinction between "singular" and "plural" forms would be called a "grammatical feature" or "grammatical category".  (This is a different use of the term "category" than in the OP.)  The term is most frequently applied to "obligatory (grammatical) categories", i.e., to those that are present in every word within its word class.  In English, nouns and some other word classes are always singular or plural, although in some cases there's no morphological[^1] or even semantic[^2] difference.  A singular noun usually has its way of forming the corresponding plural form, but sometimes it doesn't[^3].

The situation can be different in other languages.  The grammatical category of "number" (i.e., singular vs. plural) can be non-obligatory (perhaps I should even say "non-existent", as a grammatical category).  In those cases a noun can be typically used without such a grammatical distinction, while there's a semantic distinction which is however generally inferred from context.  Sometimes the context is insufficient to determine whether the speaker is referring to a single member of a "category" (in the sense of the OP) or to many members of the category; and the language provides an optional lexical or grammatical mechanism that can disambiguate this when needed.

Word making generally consists not just in determining the form and the meaning of a new word, but also in setting some precedent as to how the new word will be used within the grammar of the target language (assigning values for grammatical categories, and possibly supplying the inflected forms).   Different word making strategies seem to come with the respective conventions for where the new word ends up grammatically.

* Loanwords: 

    * If the grammatical category works very similarly between the source and the target languages, its value can be borrowed along with the form of the word.
    * Otherwise, analogy with previously borrowed formally similar words is applied.

* Composition:

    * The head word will probably determine the grammar for the new compound word.

* Derivation:

    * Each productive derivation affix either determines the grammatical values and inflected forms of any derivation result, or it preserves the grammar of the lexical input to the derivation.  For example, if the language expresses the grammatical category of number through grammatical endings, then suffixes will typically determine the grammatical number of words derived by suffixation, while prefixes or infixes will not alter the grammatical number of the root: "quasipeople" are plural because "people" are.

* Onomatopoeia, conversion, acronyms,...

    * A compromise between the form (to allow inflection) and the meaning (to allow straightforward substitution for previous used "synonyms") will be sought, so that the new word is easy to use as per its word class.

And so on.  

The details of this are language dependent and academical grammars of inflected languages tend to include voluminous chapters on word formation.

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However, if you take a language like Thai, there's no reason to dwell on whether a new word is singular or plural, because it is, in its base form, both - or neither, as you please.  That's quite different from English.  We'd say that the Thai noun is normally *unmarked* for number.  It is possible to mark the number when constructing a sentence, but that's not the task for the maker of a new word.

I think that Mandarin and Thai are rather similar in this respect.


[^1]: "a fish" vs. "two fish"

[^2]: "The committee has" vs. "The committee have"

[^3]: "music"