A Monarch butterfly garden may consist of just a single plant, there are no rules or plant police that require you to any more than you have the room or the budget for. I grew the common milkweed to the left in a 4"x4" pot. It is remarkable how large a milkweed can grow in a small area. While I wouldn't recommend restricting your milkweed to such a pot - it demonstrates that even modest pots with adequate drainage will suffice. 
Ironically, growing an isolated plant such as this will be the target of many female Monarchs to lay their eggs. In fact, I've observed  female Monarchs prefer plants that are isolated or are not overly crowded. Considering Monarch's can have a wing span up to 4" - it is important to remember that allowing females to navigate around the plant will increase the number of eggs deposited.