You have already set an alarm once, so you know how to do it. The key is to set a second alarm, whose purpose is to end the original alarm. The difficult part is finding the original alarm to cancel, however, you can just execute this code upon your second alarm to cancel the first, using the cancel pending intent. This code, as Android docs specify, will cancel all alarms with the same class. Thus, you only need to create an intent, pass it to the AlarmManager, and you can cancel the alarm.
Intent alarmintent = new Intent(parent.getContext(), Alarm_Receiver.class);
PendingIntent sender = PendingIntent.getBroadcast(parent.getContext(), CountMultipleAlarm,
alarmintent,0);
am.cancel(sender);