Database
[{"_id":ObjectId("4d663451d1e7242c4b68e000"),"topic":"abc","subLevel":{"id":1}},{"_id":ObjectId("4d6634514cb5cb2c4b69e001"),"topic":"bce","subLevel":{"id":1}},{"_id":ObjectId("4d6634514cb5cb2c4b70e002"),"topic":"bec","subLevel":{"id":2}},{"_id":ObjectId("4d6634514cb5cb2c4b70e003"),"topic":"vvv","subLevel":{"id":3}}]
Query
db.collection.aggregate([{"$match":{"subLevel.id":{$in:[1,2]}}},{$group:{"_id":"$subLevel.id",ids:{$push:"$_id"}}},{$project:{_id:false,ids:{$size:"$ids"}}}])