Anything above 3 hosts requires "default" licensing, i.e. you need a license for vCenter Server, and the licenses for the hosts (per CPU socket licensing with a 32cores/socket maximum). Cost depends on your needs, and the features you want.
For vCenter Server you can use "vCenter Server Foundation", which supports up to 4 hosts, or "vCenter Server Standard", which has no such license limits (current technical maximum = 2500 hosts). For ESXi you can choose between a "Standard", and an "Enterprise Plus" edition, depending on your requirements.
Hint: VMware offers different "Acceleration Kits", which contain either vCenter Server Foundation, or Standard, and a number of CPU licenses at reduced cost. All you then need is to add additional CPU licenses to cover your hosts.
All of these options are of course much more expensive than an Essentials Kit, so that it may be worth considering a CPU and/or RAM upgrade for the existing hosts. Anyway, please contact a VMware partner in your area to get an offer.
Also note that all of these option also require that you purchase Subscription&Support (SnS) for at least 1 year.
André