The bottle is 30 mL and the serving is a filled dropper, so servings depend on the dropper size.
Divide 30 mL by the mL in the recommended dropper fill. If a full dropper is about 1 mL, that is roughly 30 servings, which fits the 30-day supply per bottle. Droppers differ, so this is an estimate.
Check the recommended serving on your current label and the servings per container before relying on days of supply or cost per day.