[{"_id":10,"topic":"abc","subLevel":{"id":1}},{"_id":20,"topic":"bce","subLevel":{"id":1}},{"_id":30,"topic":"bec","subLevel":{"id":2}},{"_id":40,"topic":"vvv","subLevel":{"id":3}}]

db.collection.aggregate([{"$match":{"subLevel.id":{"$in":[1,2]}}},{"$group":{"_id":"$subLevel.id","count":{"$sum":1}}}])