How much food do you need for your caterpillars? A LOT! Caterpillars have one purpose in life; EAT.
It is hard to tell you how much plant or how many leaves you need. Plants grow with small leaves, large leaves, multi stemmed, few stems, and that is only from the same species of plant. Once you start talking about different species of butterfleis, the number and size of leaves vary much more.
Monarchs and Milkweed:
Tropical milkweed - a multi stemmed plant (8 or more stems) in a one gallon pot about 12 inches tall SHOULD feed 1 to 1 1/2 caterpillars.
Common Milkweed - quite a few large leaves will be needed to feed one caterpillar.
Gulf Fritillary - Zebra Longwing and Passionvine:
Large leaf passionvine - one full plant with several stems and many leaves in a four inch pot should feed one caterpillar.
Painted Lady and Hollyhock:
One full plant in a six inch pot should feed one caterpillar.
Painted Lady and Plantain (not the banana plantain)
One full four inch pot should feed two caterpillars.
One full six inch pot should feed three or four caterpillars.
Six Inch Pot
Should feed one caterpillar
Ribgrass or English Plantain
Four Inch Pot
Left; should feed two caterpillars