[{_id:1,id:1,name:"User1",notifications:[{id:1,read:false},{id:2,read:false},{id:3,read:false},{id:4,read:true},{id:5,read:true}]}]

db.collection.find({id:1},{_id:0,name:1,notifications:{$slice:20},notRead:{$size:{$filter:{input:"$notifications",cond:{$eq:["$$this.read",false]}}}}})