Database
[{"_id":ObjectId("63a994974ac549c5ea982d2b"),"title":"Destroyer","year":2018},{"_id":ObjectId("63a994974ac549c5ea982d2a"),"title":"Aquaman","year":2014},{"_id":ObjectId("63a994974ac549c5ea982d29"),"title":"On the Basis of Sex","year":1998},{"_id":ObjectId("63a994974ac549c5ea982d28"),"title":"Holmes and Watson","year":1940},{"_id":ObjectId("63a994974ac549c5ea982d27"),"title":"Conundrum: Secrets Among Friends","year":1957},{"_id":ObjectId("63a994974ac549c5ea982d26"),"title":"Welcome to Marwen","year":2000},{"_id":ObjectId("63a994974ac549c5ea982d25"),"title":"Mary Poppins Returns","year":1997},{"_id":ObjectId("63a994974ac549c5ea982d24"),"title":"Bumblebee","year":2018},]
Query
db.collection.aggregate([{$setWindowFields:{sortBy:{year:-1},output:{maxYear:{$max:"$year",window:{documents:["unbounded","current"]}}}}},{$match:{$expr:{$lt:[{$subtract:["$maxYear","$year"]},20]}}},{$group:{_id:0,count:{$sum:1}}}])