About 1 frog per 5 gallons in a vivarium tank is good.
Don't mix species or morphs, at least at first. Get some experience before you decide whether you really want to do it. Mixing is an on-going argument. Just enjoy your first darts without getting into this. Morphs and other stuff is for later.
Darts are actually pretty easy to keep, and less hassle in the long run because you can set up a biological system that doesn't require constant tearing apart and sanitizing.
You want a terrarium tank with live plants, so this is the main issue-- perhaps waterfalls, water features and such. None of this really has to be high tech, but there are many ways to do it. It helps to be as interested in the plants and how to grow tropical plants in a terrarium as it does to be interested in the frogs. They go together and prosper together.
Look around for care sheets. Saurian is a good source, but there are a dozen others. Lurk around and find out what you want to do and do it. Many dart keepers have web sites and instructions on building dart vivariums.
I just did my original tank on my own-- a paludarium, with fish in the aquarium section and D. galactonotus on land, that is still working well five years later. I kind of invented it for myself, I guess, although it isn't original at all. I now have about 12 different set-ups for various dart frogs. Some have worked out better than others. None have involved anything that requires a lot of technical knowledge. None are automated, and all require daily attention.

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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus