That depends on what software license you have. Our software subscription license model allows you to activate on multiple Access Servers at the same time. They will then share this subscription. In that situation you only need one subscription for the whole group of nodes. If you still use the fixed license activation key model, then these keys only unlock connections on a single server, and these keys cannot be shared across multiple servers. In that situation you would need a separate key for each node.
In the case of subscription, the sharing of allowed connections occurs in the following way. As long as the total amount of connected VPN clients on all Access Server nodes that use the same subscription does not exceed the allowed amount of VPN connections on the subscription, then each node individually will be allowed to connect the full amount of allowed VPN connections on the subscription. Only when that total goes over, will the subscription system eventually lower the amount of allowed VPN connections just enough so you will again be within the limits.
As an aside, we do want to point out that load balancing using round-robin DNS with clustering is supported. But load balancing use a type of device or service that stands inbetween the Access Server and the OpenVPN clients can be troublesome and is not supported. If you are considering doing this, try to setup sticky sessions at least, as it may help with some authentication problems.