Last time i checked you combine and then can "break up" any licenses you have within the my vmware portal (if you have the rights to do so for those licenses). So you could have 30 x single vcenter license and a 30 licenses for x number of hosts (depending on how many hosts per vcenter).
The best approach would be to talk to your VMware TAM on this.
Thanks for the response, i just also heard from vmware on this topic. In a nutshell this is what they said.
Please be informed that the single license key that you obtain upon combining, can be used on all ESXi hosts as well as for vCenter server. As long as the assigned quantity of the license key does not increases the total capacity of the license key, you will be under license compliance.
As I understand it we are free to have a single license key for vcenters , associated with as many vcenters as the license allows us.
The same would apply to ESXi licenses, so we can associate the ESXi license with whatever hosts we like (via vcenter or other means) as long as the CPU count is below the number of CPUs it was issued for