Database
[{"type":"A","item":"I1"},{"type":"A","item":"I2"},{"type":"A","item":"I2"},{"type":"A","item":"I2"},{"type":"A","item":"I3"},{"type":"B","item":"I4"},{"type":"B","item":"I4"},{"type":"B","item":"I5"}]
Query
db.collection.aggregate([{$group:{_id:{type:"$type",item:"$item"}}},{$group:{_id:"$_id.type","item-count":{$sum:1}}}])