You need a vSphere licence for each ESXi host running any VM, plus a vCenter Server licence for each instance of vCenter which you use to manage your ESXi hosts.
Pretty sure vCenter Server Foundarion only allows a vCenter to manage 3 hosts maximum.
-------------------------------------------------------------------------------------------------------------------------------------------------------------
Although I am a VMware employee I contribute to VMware Communities voluntarily (ie. not in any official capacity)
VMware Training & Certification blog