ONE / ONES

One

One is a substitute word. We often use one instead of repeating a singular countable noun.

• ‘Which is your boy?’ ‘The one in the blue shirt.’
• I want that one, not this one.
• ‘Can you lend me a pen?’ ‘Sorry, I haven’t got one.’

One has a plural ones.

• Green apples often taste better than red ones.

http://learnenglish.britishcouncil.org/en/english-grammar/pronouns/one-and-ones

http://www.better-english.com/grammar/one.htm

http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-58489.php