[{"_id":"624713340a3d2901f2f5a9c0","username":"fotis","exercises":[{"_id":"624713530a3d2901f2f5a9c3","description":"Sitting","duration":60,"date":"2022-03-24T00:00:00.000Z"},{"_id":"6247136a0a3d2901f2f5a9c6","description":"Coding","duration":999,"date":"2022-03-31T00:00:00.000Z"},{"_id":"624713a00a3d2901f2f5a9ca","description":"Sitting","duration":999,"date":"2022-03-30T00:00:00.000Z"}],"__v":3}]

db.collection.aggregate([{"$match":{"_id":"624713340a3d2901f2f5a9c0"}},{"$project":{"username":1,"exercises":{"$filter":{"input":"$exercises","as":"exercise","cond":{"$eq":["$$exercise.description","Sitting"]}}}}},{"$addFields":{"count":{$size:"$exercises"}}}])