[{"_id":1,"User":"ABC1",likes:[1123,123,123]},{"_id":2,"User":"ABC2",likes:[1123,123]},{"_id":3,"User":"XYZ1",likes:[]}]

db.collection.aggregate([{$project:{User:1,numberOfLikes:{$cond:{if:{$isArray:"$likes"},then:{$size:"$likes"},else:"NA"}}}}])