This is how many animals the buyer actually took. Usually it matches the lot, but if a lot is split at the ring — a buyer takes 18 of 20 head — this records the count for this transaction.
For per-head lots this number directly drives the gross amount. Recording it on the transaction (rather than reusing the lot's count) is what makes split sales possible without rewriting the lot.
Technical section
Data model, API, and internals — visible to developers.