[{_id:"0",cluster:1},{_id:"1",cluster:1},{_id:"2",cluster:5},{_id:"3",cluster:5},{_id:"4",cluster:4},{_id:"5",cluster:5},{_id:"6",cluster:5},{_id:"7",cluster:1},{_id:"8",cluster:5},{_id:"9",cluster:1},{_id:"10",cluster:5},{_id:"11",cluster:3},{_id:"12",cluster:5},{_id:"13",cluster:2},{_id:"14",cluster:4},{_id:"15",cluster:1},{_id:"16",cluster:5},{_id:"17",cluster:3},{_id:"18",cluster:5}]

db.collection.aggregate([{$group:{_id:"$cluster",count:{$sum:1}}},{$project:{"_id":0,"cluster":"$_id","count":"$count"}}])