I want to write my web content in about six paragraphs. Should I contemplate on writing them by incorporating keywords at least twice in each of them? If I have to include keyword only once what should be the size of a paragraph?
You should not incorporate them twice in each of them. That's too much. If possible don't use them more than 3-4 times on the whole page, except your article is really long. In this case keep KW density between 1-1.5%.