category to indicate (usually) more than one thing, or (in some language
s) more than two, or (very rarely) more than three.
All languages have a concept of plurality, though quite a lot only mark it in the pronouns. I don't know of any language without distinct words 'I' vs 'we', or 's/he' vs 'they'. (I'd be interested to hear if you've got one.) Oddly though, it is quite common for "you" to be both singular and plural. In Chinese they are marked with suffixes: wo~ 'I', wo~men 'we'; and likewise in Japanese: watasi 'I', watasitati 'we'. This is unusual in that normally 'we' is a wholly different word from 'I'. These two languages don't mark plural on nouns, adjectives, or verbs.
In Indonesian plurals are optionally marked by reduplication: if you want to emphasize the manyness or the disparateness of the 'people' you may say orang-orang.
Bantu languages have class prefixes, and these change into different prefixes for the plural: for example in Swahili words beginning with ki- change it to vi-, humans beginning with m- change that to wa-, and so on through a number of others. One class is the same in singular and plural (n-), and another has a prefix only in the plural (ma-).
Polynesian languages typically lengthen one of the internal vowels or use partial reduplication. In Samoan a few nouns and many adjectives have plural forms, e.g. tele 'big', plural tetele. in Maori some kin or human nouns lengthen the third-last syllable, and one word tamaiti 'child' has a distinct plural form, tamariki.
Semitic languages have many so-called broken plurals, formed by internal alteration of vowels according to a number of patterns, e.g. Arabic kitaab 'book', plural kutub, but su?aal 'answer', plural ?as?ilah. They also have suffixes for many nouns and adjectives.
The European method is generally to use suffixes. English has almost completely regular plurals, with the three forms -s -es and -z (spelt -s) depending on pronunciation, and French mainly adds an -s that is silent in most circumstances. But these have simplified a great deal from the older Indo-European method, in which a single ending rolled up the number, the case, and the gender.
I have been mainly speaking as if number is a category of noun and (what often goes with it) adjective, but verbs also commonly inflect for singular and plural subject or (less commonly) direct object, or even (as in Basque) indirect object. I think it would be relatively rare to inflect verbs and not nouns for number, but examples may be found in an Aboriginal language such as Bidyara.
Germanic and Celtic languages also use internal vowel change in some of their plurals, the phenomenon of umlaut: in English it is now restricted to a few like man ~ men, mouse ~ mice, but it is common in German, and to a lesser extent Welsh, and in both these it may be just umlaut (bardd ~ beirdd 'bards'), or umlaut with an ending.
In Greek a neuter plural noun takes a singular verb. This derives (it is believed) from the fact that the neuter plural ending -a was a collective singular -H in very early proto-Indo-European.
Quite a few languages (Old English, Greek, Sanskrit, Arabic, Bidyara, to name but a few) have a dual, for two things or people, distinct from the plural, which is therefore restricted to three or more. Some languages -- the only ones I know of are in Melanesia, such as Bislama and Fijian -- even have a trial, for three people or things.
This is, of course, the sketchiest of surveys, but I hope to have touched on the general principles of pluralization around the world.