[{"reactions":{"👊":["5f3c569e70441a0017511fe2","5f88d49dbf1b850017e28f4e","5f488189b0c5970017aa8c7a"],"😋":["5f539086cfb52a00176089ed"]}}]

db.collection.aggregate([{$addFields:{reactionsArray:{$objectToArray:"$reactions"}}},{"$project":{reactionsSize:{$reduce:{input:"$reactionsArray",initialValue:0,in:{"$add":["$$value",{$size:"$$this.v"}]}}},},}])