Breastmilk supply

What are signs that your milk supply is low? LO has normal wet/poopy diapers. She seems satisfied. I currently only pump once a day so that I have milk on hand in case I'm running errands. When I do pump, I only get about 1-2 ounces per breast so a total of anywhere between 2-4 ounces total. Since LO has normal wet diapers, I've assumed my supply is fine but I want to know if this is actually low and what I should do to increase it.