Hi and welcome to the communities,
you need 1 vCenter Server Standard license, which can manage up to 1000 hosts.
For your ESXi Hosts you will need Standard license or above (Enterprise, Enterprise PLUS)
You can compare the license editions here: http://www.vmware.com/de/products/vsphere/compare
But since you don't use any of the cool features (you should ) Standard license for the hosts will be the cheapest.
Thank you Sir.