I like to raise the caterpillars in separate containers, to minimize the chances of any pathogen crossing from one individual to the next. Butterflies are social, I know, but I think they can make up for lost time when they get their wings. They don't have to socialize when they are caterpillars, that time is for eating milkweed and getting bigger...At least, that is what I tell myself. Sometimes I raise them in open jars, and sometimes they visit each other, when I am not looking. So I guess that even caterpillars are social animals, given a chance.

I have watched cats "butt heads" over a leaf both were eating, so I doubt they are that social. You can raise them separately or in small groups with no problem.
It is important, however, to keep your cages as frass free as possible and to check and/or wash all incoming Milkweed for predators or signs of disease.