[{"ArticleName":"Example Article","Comments":[{"Text":"Great Article","Responses":[{"Text":"No it isnt","Responses":[{"Text":"Yes it is"}]},{"Text":"Spot on"}]}]}]

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":["$$dd.Text",undefined]},1,0]}}}}]}}}}]}}}}}}])