[Pumilo]

luecs are terrestrial, meaning they dont climb that much. An 18x18x24, maybe 3-4. That tank's floor area is very similar to a 10 gallon, but a big bigger. Do your research. There are a few darts that climb, but are more expensive. For example Dendrobates reticulatus are arboreal, and you could probably have a few more. Good luck!

Please do NOT consider Reticulatus. They are by no means, a beginners frog. There are thumbnails that could be done by a beginner like Imitators or Vents, but Retics should NOT be considered for a first frog.
Luecs are terrestrial frog but they do much more climbing than others. Still, your footprint (floor space) is pretty small for 5. I would stick with about 3 myself.
If you wanted to push it up to 4, how about something a step smaller like a tricolor. They would also do a good bit of climbing.