There can be up to 12 user accounts on a single Wii U console. And whatever one user downloads from the Nintendo eShop is shared across all users on the system.
Demos are no exception. So far, all downloadable Wii U demos have a limited number of uses, which are shared among all users. This means that, for example, because the demo of FIFA 13 has only 10 uses, and if you have 12 user accounts on your Wii U, not all users will be able to experience that particular demo.