If more than one creature dies at the same time, Weatherlight Compleated’s triggered ability will trigger once for each of those creatures. As each of those resolve, its controller will take the instructed actions. For example, if it has no phyresis counters on it and seven creatures die, its controller will put a counter on it and scry 1 as each of the first six instances of the ability resolve, then put a counter on it and draw a card as the last instance resolves.