I think the supply and variety of magical items is closely linked to how pervasive magic use in the realm. When magic itself is rare, ancient relics and mystical items are also rare, but in a more magical world, empowered items become much more common. There are a few ways to counteract this though, like making the magical items require great investment of time or energy in order to craft, just as the finest swords or armor require expert hands to smith. This helps to make a greater stratification of gear, where common enchanted items are still present, say those which might inflict a fire damage over time effect in addition to normal damage through armor, while keeping a more powerful effect like Necrosis onto a dagger which has been forged from the iron reclaimed by an entire battlefields worth of blood. There are a lot of ways to keep items scarce and mysterious, for economic or lore reasons, but maintaining game balance without being too controlling a DM is still the real challenge.