from http://englishplus.com/news/news1201.htm
When are apostrophes used to make words plural?
The simple answer is this: virtually never.
I only add one in when it is absolutely necessary. Ex: do's and don'ts. Otherwise it would look like dos and don'ts... dos as in disk operating system or denial of service dos.
Fun with plurals.
I liked the 1990s.
I have many DVDs.
Laptops are great.
The dresses are on sale.
I have A's and B's on my report card.
My Y's look like elongated S's.
