Count all objects in array and infinite sub-arrays

You can try below aggregation

Basically you have to loop over the each array using $map and count for the fields where Text is not equal to $ne undefined

db.collection.aggregate([  { "$project": {    "commentsCount": {      "$sum": {        "$map": {          "input": "$Comments",          "as": "cc",          "in": {            "$add": [              { "$cond": [{ "$ne": [ "$$cc.Text", undefined ] }, 1, 0 ] },              { "$sum": {                "$map": {                  "input": "$$cc.Responses",                  "as": "dd",                  "in": {                    "$add": [                      { "$cond": [{ "$ne": [ "$$dd.Text", undefined ] }, 1, 0 ] },                      { "$sum": {                        "$map": {                          "input": "$$dd.Responses",                          "as": "ee",                          "in": { "$cond": [{ "$ne": [ "$$ee.Text", undefined ] }, 1, 0 ] }                        }                      }}                    ]                  }                }              }}            ]          }        }      }    }  }}])

Output

[  {    "commentsCount": 4  }]