[{_id:1,hotelcode:"a",availdates:["2020-01-02","2020-02-03"]},{_id:2,hotelcode:"a",availdates:["2020-02-03"]},{_id:3,hotelcode:"b",availdates:[]},{_id:4,hotelcode:"b",availdates:["2020-01-02"]},{_id:5,hotelcode:"c",availdates:["2020-01-02","2020-02-03"]}]

db.collection.aggregate([{$match:{$expr:{$gt:[{$size:"$availdates"},0]}}},{$group:{_id:"$hotelcode",total:{$sum:1}}},{$project:{_id:0,hotelcode:"$_id",total:1}}])