According to
Bitmain's support site, you can only use one coupon per miner (but if you have 2 miners in checkout, you can use 2 coupons, and so on) and the value of the coupon usually can be 50% of the miner's sale price at most:
One coupon could buy one miner. (For example, if you have two miners in one order, you could add two coupons in it)
For the S9i that Phil linked, the coupon can be 50% of the value at most, but depending on what you are buying, the restrictions are different, such as for the Aug-Sep Z9 Mini batch:
Coupons value 420 USD and below may be used for this batch.