Not unlucky, I've seen as few as one or even none (once), and as many as a dozen or more. For me the average seems to be needing two or three dungeon expeditions to get to at least five, preferably a dozen, to start with. Five is workable, just build kiln first, make 6x as much coal as the amount of bronze you figure to get early on, then deconstruct it and build a smelter.