One API call is used per request made, meaning when an email address or website domain is queried against any of the Enrichment API endpoints. A personal email, a corporate email, or a website domain lookup would all count equivalently as one API request. API requests can be by way of direct access (ex. cURL command), your own application, or via a prebuilt integration (ex. Salesforce or Marketo).
Clearbit only counts requests which return 200, 201, 202, or 404 status codes. You can see what the error codes translate to here. In addition, only unique requests are counted each billing period - meaning if 'alex@clearbit.com' were queried two days in a row, it would still only count as a single API request.
For Enrichment API details, visit our docs page.