Where are you and what milkweed did you plant? I'm thinking you have the Asclepias curassavica (tropical) and not one of the perennials species. The tropical fairly easy to start from seed on the windowsill, others need a chilling period.
I start plenty of the tropical and Ascepias incarnata (rose or swamp). (latter needs chilling). This for selling starts at the farmers market. Assuming you have tropical, I wait until it has several true leaves, and the roots showing out the bottom of the peat pellet. Once there, pot up to 4" pots, and for ease and because it works, use Miracle Grow potting mix. At this stage need dappled shade, shade cloth or something until stronger, and weather predictably warmer. Our frost is mid April, even though we've had some 70's. When potting the peat pellet, I tear down the sides a bit, and have it deep enough to put dirt over top.
How many in a pot?! I'll put 3 -4 in a 16" pot on the deck, mostly sun, 2 in a 12" pot. The plant sends up more stems and it will be full. I have them elsewhere around the yard, either starters to sell or ones I've potted. Even part shade works!
Now, back to your sicky ones. Looks like you have the cooties from H. Perhaps just trash them if they haven't recovered in more shade. I have several plants that get this, though haven't had them on the milkweed. A couple I keep trying thinking I'll get different results are Mountain mint and asters.
It's a learning process, challenging and fun!