Singular:
* One
* Two
* Three
* Hundred
* Million
* Billion
* Dozen
* Score (meaning 20)
Plural:
* Ones
* Twos
* Hundreds
* Millions
* Billions
* Dozens
* Scores (meaning groups of 20)
Note: Numbers can also be used as adjectives, describing the quantity of another noun:
* Two cats
* Five minutes
* A dozen eggs
* A hundred people
Let me know if you'd like more examples or have any other questions!