So the configured feature just modifies the maximum size of the veins. The placed feature determines how many spawn, at what locations, etc. If you want to reduce the size of the vein, modify the number in the configured feature. If you want to reduce the number of veins, modify the placed feature. The placed feature that modifies the number of veins should usually be the first one on the list. The Minecraft wiki has an entire section on what each type does.