Problem

Is there a way to check how many entities are filtered out due to mapping filters so that it is confirmed that the copy task is running successfully? 

Solution

It is possible to start the DTSS copy task with "uploadEnabled: false" and filter(s) from mapping in the task body like: 

{
    "dataTenant": "{{datatenant}}",
    "customerTenant": "{{tenant}}",
    "entityTypes": [
        "configuration/entityTypes/HCP"
    ],
    "relationTypes": [],
    "entitiesFilterInDtMetadata": {
        "configuration/entityTypes/HCP": "equals(attributes.FirstName, 'John')"
    },
    "uploadEnabled": false
}

In this case, when the task will finish you will see the number of processed entities in the "scannedDtEntities" counter.