The term number refers to whether a noun is singular or plural. Most nouns can be either singular or plural, depending on whether you are talking about one thing or more than one. You know the basic rule of adding ‐s to make the plural of a noun ( one cat, three cats), and you also know that many nouns don't follow that rule—for example, sheep (singular), sheep (plural); enemy, enemies; wharf, wharves; hero, heroes; goose, geese, and so on. Check a dictionary if you're not sure how to spell a plural noun. Do not add an apostrophe + s to a singular form to make it plural, even if the noun is a family name: the Taylors, not the Taylor's; donkeys, not donkey's; taxis, not taxi's.
The singular and plural forms of some nouns with Latin and Greek endings can cause trouble. The noun data, for example, is plural; datum is the singular form. Although today the plural data is widely used as a singular noun, you should keep the distinction, particularly in scientific writing.
The final datum (singular) is not consistent with the preceding data (plural), which are positive.
Here are some examples of Latin and Greek singular and plural words that can be troublesome: bacterium, bacteria; criterion, criteria; medium, media; alumnus (masculine singular) , alumni (masculine plural), alumna (feminine singular), alumnae (feminine plural).