No. You can obviously use one FTP account to manage more than one domain. Each domain can be assigned to a sub directory of your choice via the control panel, let's say you set domain1.com and domain2.com to the directories /blah/domain1.com and /blah/domain2.com. Set up an FTP account for /blah and there you go. That's only necessary if you want to resell of course. Otherwhise one FTP account will suffice for managing an arbitrary number of domains. Apart from that you'll also have SSH access (one account).