It may be attached to your level but the farther I got in the story line the more there were. Survival took me a little ofver 24 hours to finish the main quests. And more and more ledgendary mobs would show up. It was a pain at lvl 17. Started on normal just to explore. There is a big differance. have only seen 6 or so ledendary mobs and lvl 22 now. But not that far into the story also.