Database
[{"_id":ObjectId("5cd8609db20663a19e2c362c"),"2017":{"JAN":[{"SYMBOL":"20MICRONS","SERIES":"EQ","OPEN":"33.4","HIGH":"34.3","LOW":"33",},{"SYMBOL":"3IINFOTECH","SERIES":"EQ","OPEN":"5.8","HIGH":"5.8","LOW":"5.55",},{"SYMBOL":"3MINDIA","SERIES":"EQ","OPEN":"11199.9","HIGH":"11233","LOW":"10861",}]}}]
Query
db.collection.aggregate([{$project:{totalJan:{$size:"$2017.JAN"}}}])