A limited quantity — used to indicate that only a fixed number of items are available (sold until stock runs out).
このコラボ商品は数量限定なので、欲しい人は早めに買ってください。
This collaboration item is limited in quantity, so those who want it should buy it early.
数量限定のセットは一人一つまでです。
The limited-quantity set is restricted to one per person.
発売当日、数量限定のパンはすぐに売り切れた。
On the release day, the limited-quantity bread sold out quickly.