Use Face++ Detect API to detect faces within images, and get back face bounding box and token for each detected face. You can pass the face token to other APIs for further processing. Detect API also allows you to get back face landmarks and attributes for the top 5 largest detected faces.
All APIs can be used for free, and you can flexibly upgrade to paid service according to your business volume by Pay As You Go service or QPS solution. SDK licensing options are also available on various devices.