This .Net Framework application shows how to connect to multiple mailboxes using EWS Streaming Subscriptions. It groups mailboxes and maintains affinity per https://learn.microsoft.com/en-us/exchange/client-developer/exchange-web-services/how-to-maintain-affinity-between-group-of-subscriptions-and-mailbox-server.
Note that the application uses a customised version of the EWS API, which is available from https://github.com/David-Barrett-MS/ews-managed-api.