actually BS said there's going to be like 30 magic systems. if we assume that the magic systems are divided evenly between the 3 Shards we know about, that means each Shard gets to fuel 10 magic systems, and 10 is like a big important number on this world.
Also, Ruin and Reservation each have their own personal magic and then there's Feruchemy which comes from both, so each Shard in Mistborn is only fueling 1.5 magic systems, so your logic doesn't work.