[{"_id":"1","title":"Harry Potter","year":2000,"author":"JK. Rowling","genres":["War","Fantasy","Drama"]},{"_id":"2","title":"Harry Potter 99","year":2020,"author":"JK. Rowling","genres":["War","Fantasy","Drama","Crime"]}]

db.collection.aggregate([{$group:{_id:"$author",genres:{$push:"$genres"}}},{$addFields:{genres:{"$reduce":{"input":"$genres","initialValue":[],"in":{$setUnion:["$$value","$$this"]}}}}},{$addFields:{countGenres:{$size:"$genres"}}}])