A Horizon Security Server can support up to 2,000 simultaneous Horizon client connections, which is half the number that the Connection Server it connects to can support. Due to this, it is important to deploy multiple Security Servers so that both capacity and availability requirements are met.
Each Horizon Security Server is a stand-alone instance; therefore, there is no specific guidance with regard to how many can be deployed. A Horizon Security Server can only be paired with one Horizon Connection Server while a Connection Server can be paired with multiple Security Servers.
Keep in mind that the more Horizon Connection Servers you deploy, the more Security Servers you will need as well. For example, if you deploy the pod-maximum of 7 active Connection Servers, and intend to pair Security Servers with all of them, you should deploy 14 Security Servers to ensure redundancy, or two per Connection Server. When designing your Horizon infrastructure you will want make all layers of it redundant, as this is the only way to ensure that the high levels of availability and performance that your end users will require.