The context doesn’t necessarily need to be the plural of the schema — in fact, contexts often contain multiple schemes. The goal of contexts is to define the “public API” that the rest of your application uses to work with those resources.

So, what kind of card data are we talking about? Playing cards? How will card data be used? Will you ever care about an individual card on its own, or will it always be a part of a deck of cards? If the latter, maybe Deck would be a useful context.

Finally: this doesn’t matter too much. Name it something, Cards for instance, and change it later when the boundaries of your application are more clearly defined. Naming things is hard, but thankfully changing names is usually not bad, especially since Elixir gives useful compiler warnings when stuff no longer exists.