Database
[{"_id":ObjectId("6451529ad254d565bd12156b"),"value":"a"},{"_id":ObjectId("6451529ad254d565bd12156d"),"value":"a"},{"_id":ObjectId("6451529ad254d565bd121570"),"value":"b"},{"_id":ObjectId("6451529ad254d565bd13156b"),"value":"b"},{"_id":ObjectId("6451529ad254d5651112156a"),"value":"b"},{"_id":ObjectId("6451529ad254d565bd12166d"),"value":"c"},{"_id":ObjectId("6451529ad254d565bd121670"),"value":"c"}]
Query
db.collection.aggregate([{$match:{value:"b"}},{$count:"count"},{$match:{count:{$lt:6}}},{$project:{doc:{value:"b"}}},{$replaceRoot:{newRoot:"$doc"}},{$merge:{into:"collection"}}])