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([{$setWindowFields:{partitionBy:{sellerId:"$sellerId"},output:{numDoc:{$sum:1,window:{documents:["unbounded","unbounded"]}}}}}])