To save cost, you can designate one set of controllers as master controller. Configure them active/standby, with primary being the licensing server and backup being the backup licensing server. Use VRRP to provide redundancy so that if master fails, AP fallback to the backup. This set will also provide licensing service to the second set who will act as locals only.
Another way of doing it is, like you said, add another controller (not 2 but just one) so that even if it fails, the controllers will have licenses available to them for 30 days. Enough time to get back the master controller. Please note that all other controllers will be local.