[{_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([{$addFields:{adlength:{$size:"$availdates"}}},{$match:{adlength:{$gte:1}}},{$group:{_id:{hotelcode:"$hotelcode"},total:{$sum:1}}}])