Depending on the efficiency of filtration, and how many and the volume of water changes you do per week.
Maybe 4 fish, not to exceed 4" as adults.
If you chose smaller species, maybe a couple more.
The word predator is a bit vague.
Most Tetras are predators, most (not all) appropriately sized cichlids would not be considered predators.
There are a number of predatory gobies that might work.