Here’s my twenty cents:
No, in my experience IxDers do not work alone. Sure it’s possible, but is it efficient? And will it result in a product that meets user needs, balances business requirements and fits within technological constraints? Not likely.
You can’t really design something and then throw it over the fence and hope for the best. Working in teams of two works quite well, but 3- 4 is even better. It’s small enough to focus on what needs to be done but big enough to have different perspectives and expertise.
For contractors and remote workers, we know there are many ways to do design work with other people without being co-located but the big thing you want to get right is communication. Regular contact, clear visibility around who’s doing what and when things are due is essential.