Definition: Number of a noun: means whether the noun is singular or plural.

•  Singular: noun names one person, place, thing or idea.
•  Plural: noun names more than one person, place, thing or idea.

Rules for Forming Plurals
1.  To form the plural of most nouns, add -s
          Examples: chairs, tables, girls, cousins

2.  When singular ends in s, ch, sh, x, or z, add -es
          Examples: brushes, torches, buses, boxes, buzzes

3.  When singular ends in o, add -s
          Examples: radios, stereos, sopranos, silos

Exceptions:
4.  For a few words ending in o, preceded by a consonant, add -es.
          Examples: tomatoes, heroes, potatoes, echoes

5.  When the singular noun ends in a -y preceded by a consonant, change the y to i and add -es.
          Examples: hobby to hobbies, cry to cries, party to parties
If the y is preceded by a vowel, do NOT change the y to i.  Just add an -s to the singular.

6.  For most nouns, ending in f, add -s.  For some ending in f or fe, change the f to v, and add -es or -s.  To be sure, look up the singular in the dictionary, it will also show the plural in the entry, especially if it doesn't follow the first rule in #5.
          Examples: roof to roofs, shelf to shelves, half to halves

7.  Some nouns are spelled the same in singular and plural.
          Examples: moose, deer, sheep, tuna

8.  Some nouns form their plural in ways that are unique or unusual.
          Examples: mouse to mice, man to men, foot to feet, child to children