Foxit Quick PDF Library is licensed on a per developer basis. So in this case you need to purchase 8 licenses (1 per developer). You can use the library in as many of your products as you like, providing that all of the products are owned and developed by your company.
For your situation, given that you need 8 licenses, we would recommend that you purchase a Multiple Developer License, as this multi-user license allows for up to 10 developers on your team to develop applications using the SDK.