If you already have a tank, then you'll need to select your frogs based upon this tank. If you're set on a certain species of frog or number of frogs for a tank, then you'll need to be prepared to buy an appropriate tank.
Generally, dart frogs will need about 10 gallons of space per frog, but this varies depending on the species, orientation of the tank, and how densely planted your tank is. A more densely planted tank can comfortably house more frogs as they will have more surface area, more visual barriers, and more hiding places. Always watch your frogs carefully for signs of aggression. Follow this guide for a mid to well planted tank:
10 Gallon: 1-2 frogs
20 Gallon: 1-3 Tinctorius/1-2 pumilio/1-3 Ranitomeya/1-4 of any other type
40 Gallon: 1-4 Tinctorius/1-4 Pumilio/1-5 Ranitomeya/1-5 of any other frogs
90 Gallon +: 1-4 Tinctorius/1-4 pumilio/1-6 other Thumbnails/1-6 auratus/1-10 phyllobates/1-10 adelphobates/1-15 epipedobates/1-10 Leucamelas
18x18x18: See 20 gallon
18x18x24: See 20 gallon
12x12x18: 0 Tinctorius/0 Auratus/0 Leucamela/1-2 Epipedobates/1-2 Pumilio/1-3 Ranitomeya
It's very important to pick an appropriate tank for your frogs. Terrestrial frogs will need more floor space while more arboreal frogs will need more height and things to climb on.
Terrestrial Dart Frogs:
It's also important to pick a tank based upon how territorial your frogs are. Tinctorius are the most territorial followed by Pumilio. No more than 4 should ever be housed together no matter the size of vivarium. Often times you will need to separate Tinctorius into pairs when adults due to aggression. All should be very closely matched so far as size and weight and introduced at the same time to the tank. Auratus are less territorial but generally do best in groups of 2-4. Ranitomeya can be somewhat territorial and generally do best in trios or groups of 5 at most. Adelphobates and phyllobates are very peaceful and territorial disputes are not common, allowing you to easily keep a group of 4-5. Leucamelas and epipedobates are the most peaceful of dart frogs and can easily be kept in groups of 4-10 or more as long as enough space is provided.
Some frogs are very bold and you'll see them at every feeding. Others are somewhat shy and will come out occasionally. Others are extremely shy and will only be visible seemingly once or twice a month. The better planted your tank, the more bold your frogs will be, but this is a general guide:
Tinctorius: Extremely Bold
Auratus: Somewhat Bold to very shy.
Amereega: Generally shy
Phyllobates: Very Bold (Terribilis) to somewhat shy (Vittatus/Truncatus)
Epipedobates: Bold to somewhat shy
Pumilio: Somewhat bold to shy
Ranitomeya: Bold to very shy
Dart frogs come in all sizes, but it's important to know how big your frog will grow to be. Tinctorius are some of the largest frogs while Ranitomeya and Pumilio are some of the smallest. Auratus, Adelphobates, and Leucamelas are somewhat smaller than Tinctorius, followed by Epipedobates, then pumilio and ranitomeya. Phyllobates range from the large Terribilis, which rivals Tinctorius, to the much smaller truncatus which are just a bit bigger than epipedobates.