[{"sourceList":[{"source":"hello world, how are you?","_id":ObjectId("5f0eb9946db57c0007841153")},{"source":"hello world, I am fine","_id":ObjectId("5f0eb9946db57c0007841153")},{"source":"Is it raining?","_id":ObjectId("5f0eb9946db57c0007841153")}]}]

db.collection.aggregate([{"$unwind":"$sourceList"},{$project:{"sp":{$split:["$sourceList.source"," "]}}},{"$project":{"sizes":{"$size":"$sp"}}},{"$group":{"_id":"$_id","count":{"$sum":"$sizes"}}}])