[{_id:1,items:[{name:"item1",status:"On"},{name:"item2",status:"On"},{name:"item3",status:"Off"}]}]

db.collection.aggregate([{$addFields:{itemOn:{$reduce:{input:"$items",initialValue:0,in:{$cond:[{$eq:["$$this.status","On"]},{$sum:["$$value",1]},"$$value"]}}}}}])