Database
[{"userInfo":{amputationInfo:["Double Symes/Boyd","Single Above-Elbow"]}},{"userInfo":{amputationInfo:["Double Symes/Boyd",]}},]
Query
db.collection.aggregate([{"$unwind":"$userInfo.amputationInfo"},{"$group":{"_id":"$userInfo.amputationInfo","count":{"$sum":1}}},{$group:{_id:null,data:{$push:{k:"$_id",v:"$count"}}}},{$project:{data:{"$arrayToObject":"$data"}}},{"$replaceRoot":{"newRoot":"$data"}}])