For a single-animal weigh this is one. For a lot or pen-group weigh it's the number of head that went over the scale together, which lets you figure an average weight per head.
When the subject is a lot or pen group, set the head count to match what you actually weighed — it should line up with the lot's head count for a full-lot weigh.
Technical section
Data model, API, and internals — visible to developers.