Build one email, and use merge tags to dynamically display customized content based on criteria you specify. For example, if you publish a newsletter about baseball, you might want to send news about particular teams to particular subscribers, based on the interests they selected on your signup form. MailChimp makes that easy.
You can reformat the content inside merge fields, too. If you want to include your subscribers’ first names in a newsletter, you can display their names displayed in title case, even if some of them provided their names in all caps.