Our system is very flexible in this regard. Each membership can have one or more accounts. Each account can have one or more sites. Each site can have one or more domain names. For many people, this means one membership, one account, one site, and one domain name. For many others, it gives them the power to be as creative as they want, and to pursue several different goals.
Whether you spread your websites across multiple accounts or keep them all in one is completely up to you. Support for multiple accounts allows people who have sites for different reasons, such as business and personal, to keep the finances separate if they choose to do so.