db={students:[{"_id":{"$oid":"632b43f1675d2ce8e93b9fd2"},"name":"john","email":"john@gmail.com","phone":{"$numberLong":"9800000000"},"password":"$2b$10$XWZdlm7ygfYqjTnSMjz.6u/DQOb2/HKgpe69B3F3V5zWTJPPGBoJO","college":"oxford","fav":[{"$oid":"632b443e675d2ce8e93b9fe1"},{"$oid":"632b4436675d2ce8e93b9fde"}],"__v":2},{"_id":{"$oid":"632b4400675d2ce8e93b9fd5"},"name":"sam","email":"sam@gmail.com","phone":{"$numberLong":"9800000000"},"password":"$2b$10$mc0Ogs7cPtzzu4Ru6TJzdOCn9cywvtmnG/9OBNyIkEh4VmnEgHwEq","college":"oxford","fav":[{"$oid":"632b443e675d2ce8e93b9fe1"}],"__v":1}],teachers:[{_id:{"$oid":"632b443e675d2ce8e93b9fe1"},name:"a"},{_id:{"$oid":"632b4436675d2ce8e93b9fde"},name:"b"}]}

db.teachers.aggregate([{$lookup:{from:"students",let:{id:"$_id"},pipeline:[{$match:{$expr:{$in:["$$id","$fav"]}}},{$project:{_id:1}}],as:"count"}},{$set:{count:{$size:"$count"}}},{$sort:{count:-1}},{$limit:1}])