Number is a grammatical category representing a count. Typically, number can be singular (one) or plural (more than one). However, some languages have additional numbers, such as dual (two, which survives in English in the word both). Other languages, such as Japanese, do not distinguish number.
In English, regular nouns form the plural with -s or -es, while irregular nouns, as well as all pronouns, have separate forms that must be learned separately. In other languages, pronouns also form plurals regularly. For example, in Mandarin Chinese, 他 (tā) means "he/him," and 们 (men) is the plural marker for pronouns and animate nouns, so 他们 (tāmen) means "they/them." (This example uses simplified Chinese characters; the traditional character for the plural ending is 們).