I have a wordpress blog and i want to know that should i use tags for posts and how much tags i can use in one post. Is pages with these tags considered as duplicate content. Please suggest .
Tags are used to help find related content on a site.
You can use as many or as few as you like.
Individual words are not considered duplicate content.
Tags are useful not only for finding related content but also they are needed to make a functional for some WP plugins such as WP Related Posts... And as regards the number of the words in tags, of course you can use as many tags for a single post as you want but many tags will confuse your visitors.
Use it as your content describtion
That's what the excerpt is for.