Mixing Dart frog species or morphs is not recommended. Based on that; you would need a separate enclosure for every desired species or morph. Dendrobates have been kept in the past on smaller set-ups; but think the smallest decorated vivarium size for them is an 18x18x18 in. ExoTerra. Once you build up the vivarium's false bottom/drainage layer and the background or sides, will see that the enclosure volume will be reduced almost by a third. If dealing with Dart species that climb; would go with a 24 in. tall enclosure.

Josh has information on his site about specific numbers according to species. Some Darts can be kept in groups while others are kept in trios of 1F/2Ms or pairs .