There is no limit to the number of sites you can have in your account. However, if you plan on adding several sites to your account, you may want to use the grouping feature in order to classify your site according to client or theme. When you add a new site, you can click on the 'Grouping Options' at the top right hand corner of the page and create your own group under which to store your site. This makes for easier sorting especially if you have many sites in your account.