A single server can easily collect sFlow from hundreds of sFlow agents. If you exceed the capacity of a single server you can distribute the agents among members of a pool of servers.
If you have multiple servers that want to monitor the same agent then you can either get the agent to send datagrams to each server. Alternatively, the analysis server can replicate the UDP datagrams and send them to additional servers. You can use sflowtool as a replicator: