A single Horizon Composer instance can support only one vCenter Server. A single vCenter Server can support up to 10,000 desktops, which is the maximum number of desktops supported in a single Horizon Pod. As a result, while Horizon can support multiple vCenter Servers, each with its own dedicated Horizon Composer instance, only one of each is actually required to manage up to the maximum number of desktops supported in a single Horizon Pod.
If you still wish to have multiple vCenter Servers in your Horizon environment, you will need multiple instances of Horizon Composer as a single instance can service only one vCenter Server. Each additional instance of Horizon Composer will also require its own database, as the databases cannot be shared between Horizon Composer instances.