4 数量要求
3句英文任你选
We'll need about 400 units per order.
每单大概400个。
Do you have any in stock?
你们有现货吗?
Please send us 50 sets only so that we may tap the market. If successful, we will give you large orders in the future.
试订50台，以开发市场。如果成功，随后必将大量订购。

半个句型要记牢
in stock (现货，有库存)

Tip:
相反的out of stock表示脱销的意思。卖方询问卖方订货数量，可以说 How much do you intend to order about the goods? (你打算订多少呢?)