Opossums are marsupials. When the joeys are born they crawl from the vagina to the pouch where they latch on to one of thirteen nipples. Even when they are developed enough to leave the pouch, they will still nurse. Because opossums don't usually keep to identifiable territories and dens, the young ride on the mother until they are about half grown.
I don't know how many joeys an opossum mother can carry, I've seen one carrying more than she could have supported in her pouch.