An API call is any request sent to the API. It may contain one or multiple name queries or screens. For example, an API request may include 15 names screened in a batch, each of which will use the same set of search parameters (e.g. type, category, or list).
However, for the purposes of billing, every individual query is counted as a call. For example, a single API request containing two names queried would be counted as two API calls. Alternatively two successive API requests, each containing a single name query, would equate to two API calls.