[{_id:1,field1:"value",field2:"A",},{_id:2,field1:"value",field2:"A",},{_id:3,field1:"value",field2:"C",},{_id:4,field1:"value",field2:"B",}]

db.collection.aggregate([{"$facet":{"first":[{"$match":{"field2":"A"}}],"second":[{"$match":{"field2":"B"}}],"third":[{"$match":{"field2":"C"}}]}},{"$project":{"A":{"$size":"$first"},"B":{"$size":"$second"},"C":{"$size":"$third"}}}])