List<Observable<SomeObj>> obsList = ....

Observable.zip(obsList, (arrayOfResults) -> todo склеить)
.subscribe(result -> System.out.println(result));

public Observable<Obj> whenAll(List<Observable<Obj>> tasks) {
    return Observable.from(tasks)
            //execute in parallel
            .flatMap(task -> task.observeOn(Schedulers.computation()))
            //wait, until all task are executed
            //be aware, all your observable should emit onComplemete event
            //otherwise you will wait forever
            .toList()
            //could implement more intelligent logic. eg. check that everything is successful
            .map(results -> true);
}