Our system allows you to target up to four keywords with a single launch, but there is no way to delegate how many units are given for each keyword. If you set up a single launch to target multiple keywords the units you use for your promotion will be evenly distributed among each keyword.
For this reason, we suggest setting up a separate launch targeting each individual keyword. The most cost-effective way to do this is to sign up for a Viral subscription. The subscription is available for $450/month for the first product and allows you to run unlimited promotions on the product.
It is also important to note that the more keywords you go after, the more inventory will be required. If you need 200 units to rank for Keyword A and 200 units to rank for Keyword B, you need to give 400 units total.