<div *ngFor="let event of events">
   <div>Eventname:{{event.name}} </div>
   <div> Amount of Arrival: {{event.arrival.length}} </div>
</div>

Event{
  a{
    date: "12,Nov"
    name: "best event"
    arrival:{
      a{
        name:"david",
        from:"USA"
       }
      b{
        name:"Mike",
        from:"GER"
       }
    }
   }
}

getDataList(listpath:string){
    this.itemsRef = this.db.list(listpath);
    this.items = this.itemsRef.snapshotChanges().pipe(
      map(changes =>
        changes.map(c => ({ key: c.payload.key, ...c.payload.val() }))
      )
    );
  return this.items;
  }

this.db.getDataList("/Event/" ).subscribe(res => {
     this.events=res;
  });