ones, one's, once

Ones or ones means more than one one--it may refer to one-dollar bills, or typed or written characters--
Can I have five ones for a five? 
Can I have 5 1's for a 5?
I remember there were three ones in the phone number.
I remember there were 3 1's in the phone #.

One's or one's is the possessive ( belonging to) form of the pronoun "one"--
One should take one's time and read the questions carefully.
One should not divulge one's personal information to strangers.
One's or one's might also be used as a contraction (short form) for "one is", but this would be uncommon, as the use of "one" as a pronoun is very formal, and the use of a contraction is informal--
One's not likely to remember something that is not important.

Most people are more familiar with the use of "one" as a pronoun in the expression "no one"--
No one was home.
I rang the bell, but no one answered.

Once or once ( pronounced wuhnss ) means only one time--
We ate there once, but didn't like it.
I met him once, a long time ago.
Once upon a time...

Now that you know that, you can say--
"One should count one's ones once, at least."

Note--"no one" may be mistaken for "noon", meaning 12 in the afternoon--
I called at noon, but no one answered.
Note #2--"once" sounds a lot like "wants", meaning desires or has a wish for--
He wants to go there, just once.
Note #3--"ounce" or "ounces" ( rhymes with pounce or pounces ) means a unit of measure--of weight or volume--
There are 16 ounces in a pound.