First off welcome aboard. Sounds like you are doing your homework, a good idea with darts. How many you can put in there will depend on the species. Size of the animal and whether they are arboreal or terestrial will dictate how many you can have along with the orientation of your vivarium. It is normally accepted that they need 5 gallons of usable space per frog. Your enclosure is roughly 67 gallons but not all is used by the frogs. The more space the better. An example is I have 4 tiny R.amazonicas in a 75g.There are many members here with darts and some of the setups are amazing. look through some member photo albums and see what ideas you can pick up. Do you know what species you want yet?