Database
[{"key":1},{"key":2},{"key":2},{"key":2},{"key":3},{"key":3},{"key":4},{"key":5},{"key":6}]
Query
db.collection.aggregate([{"$addFields":{"CheckField":{$switch:{branches:[{case:{$eq:["$key",1]},then:"IsOne"},{case:{$eq:["$key",2]},then:"IsTwo"},{case:{$eq:["$key",3]},then:"IsThree"}],default:"Other"}}}},{$group:{_id:"$CheckField",Total:{$sum:1}}}])