Database

[{"_id":"635d40803352895afffdc294","count":3,"student":[{"_id":"635d40803352895afffdc294","name":"D R","email":"d@gmail.com","number":"9198998888","rollNumber":202,"departmentID":"635a8ca21444a47d65d32c1a","classID":"635a92141a081229013255b4","position":"Student"}]},{"_id":"635eb8898dea5f437789b751","count":4,"student":[{"_id":"635eb8898dea5f437789b751","name":"V R","email":"v@gmail.com","number":"9198998899","rollNumber":203,"departmentID":"635a8ca21444a47d65d32c1a","classID":"635a92141a081229013255b4","position":"Student"}]}]

Query

db.collection.aggregate([{"$group":{"_id":null,"totalCount":{"$sum":"$count"},"docs":{"$push":"$$ROOT"}}},{"$project":{"docs":{"$map":{"input":"$docs","as":"doc","in":{"$mergeObjects":["$$doc",{"totalCount":"$totalCount"}]}}}}},{"$unwind":"$docs"},{"$replaceWith":"$docs"}])

Result