One plant is enough to sustain one egg laying event. Not all the eggs that hatch will live to be chrysalis forming caterpillars anyway. Growing up here, the native milkweeds were abundant in the abandoned fields and pasture. As a kid I was always searching for the monarch cats and any chrysalis I could spot. Even then, before the monarch population was reduced, you'd find the caterpillars on just a few milkweeds. Some didn't have any. Monarchs come through here so seldom now that some years I don't see any. The milkweed is still here, but it's uncommon to find the caterpillars on any plants now.
