Database
[{"sellerId":1,"rest":"test"},{"sellerId":2,"rest":"test"},{"sellerId":1,"rest":"test"},{"sellerId":2,"rest":"test"},{"sellerId":1,"rest":"test"},{"sellerId":2,"rest":"test"},{"sellerId":1,"rest":"test"},{"sellerId":2,"rest":"test"},{"sellerId":1,"rest":"test"},{"sellerId":2,"rest":"test"},{"sellerId":1,"rest":"test"},{"sellerId":2,"rest":"test"},]
Query
db.collection.aggregate([{"$group":{"_id":"$sellerId","numDocs":{"$sum":1},"docs":{"$push":"$$ROOT"}}}])