I have a sourdough starter question.

The usual method is to discard half of your starter, and add the equivalent amount in water and flour back in its place. So if you remove 1 cup of starter, it gets replaced with 1/2 cup water and 1/2 cup flour. You can use the discard amount in a variety of ways, by the way- so don't feel obligated to actually discard it down the sink. If you didn't remove part of your starter, you would need to use ever increasing amounts of flour and water to feed it and keep it alive- and end up with an ever increasing amount of starter. Pretty soon you would have a monster on your hands, who required pounds of flour to feed. And along with that, you'd need a lot larger jar to keep it in. You have a living creature in that jar, and controlling the feed is the only way to control the size it grows to.