[{"sourceList":[{"source":"hello\nworld, how are you?","_id":ObjectId("5f0eb9946db57c0007841153")},{"source":"hello world, I am fine","_id":ObjectId("5f0eb9946db57c0007841153")},{"source":"Is it raining?","_id":ObjectId("5f0eb9946db57c0007841153")}]},{"sourceList":[{"source":"Look at the beautiful tiger!","_id":ObjectId("5f0eb9946db57c0007841153")},{"source":"Is it raining?","_id":ObjectId("5f0eb9946db57c0007841153")}]}]

db.collection.aggregate([{"$unwind":"$sourceList"},{$project:{"sp":{$reduce:{input:{$split:["$sourceList.source","\n"]},initialValue:[],in:{$concatArrays:["$$value",{$split:["$$this"," "]}]}}}}},{"$group":{"_id":null,"elements":{$addToSet:"$sp"}}},{"$unwind":"$elements"},{"$project":{"sizes":{"$size":"$elements"}}},{"$group":{"_id":null,"count":{"$sum":"$sizes"}}}])