Challenge Hound supports both set goals for challenges, as well as open-ended challenges. By default, challenges have a fixed goal. For example, run 30 miles in 30 days. In these challenges, activities are added until the goal is reached.
In contrast, open-ended challenges have no fixed goal. An open-ended challenge will keep adding applicable activities to a challenge until it has ended. There is no fixed goal, it is accomplish as much as possible before the challenge ends.
To make a challenge open-ended, simply click the "open-ended" button when creating or updating a challenge: