Hi Vahid –
The difference here is really whether you are opening change streams with the driver (as it sounds like you are today) or are using Realm (ex. with one of the Realm SDKs/Sync or Triggers).
When opening change streams directly, each one will count as it’s own connection from the client/server to Atlas.
However, Realm has built-in proxying of change streams across clients, giving some scalability benefits for change streams that you would otherwise need to write yourself in an interim layer between Atlas/change stream consumers. For this reason, when using Realm the change stream/end-user limits are structured slightly differently. When using Realm, generally the number of change streams open will count against your Atlas limit but you will be able to have far more concurrent clients due to the proxying.
Hope that helps!
Drew