[{"array":[0,1,2]},{"array":[2,3,4]},{"array":[0,1,2]},{"array":[3,1,2]},{"array":[4,0,2]},{"array":[1,1,3]},{"array":[0,2,1]},{"array":[2,3,4]},{"array":[0,1,2]}]

db.collection.aggregate([{$group:{_id:"$array"}},{$group:{_id:null,sum:{$sum:{$size:"$_id"}}}}])