As long as an account balance is positive, you may create and manage as many assets (web sites, domains, databases, etc) as you like funded by that account.
You should only ever have one active membership (because the membership represents "you" as an individual). Creating or accessing multiple memberships is a violation of our Terms and Conditions of Service. Doing so will cause all sorts of problems, and may result in substantial additional fees if we have to step in. If you've already done this accidentally, and for some reason we haven't detected it, please send us Transfer assistance requests from both memberships to consolidate your accounts onto one membership.
Within your one membership, you may have multiple accounts (each of which represents "a discrete pool of money") if you wish. We do not ever require you to have more than one account.
The ability to create additional accounts does not have any technical effect in our system. It is solely a convenience feature for you that allows you to differentiate ownership of funds and assets, help track/control your spending, and share things when there are multiple people involved in a project.
Often, this is done for tax, business, or legal purposes, such as to keep business and personal sites separate, or to segregate expenses if you need to manage sites or domains for clients, organizations, etc. It can also be useful if you have one site that is very active or given to surges and another site that must remain available at all times. If both sites are on the same account and the account runs out of funds, both sites will be disabled. If they are on separate accounts, the busy site can never cause the other site to become disabled.
Therefore, you never have to create a second account for yourself to host an additional site, but you do have that option if it suits your needs.
For more information about setting up services that will be shared with others, see this related FAQ entry.