You can probably put about 3 in a tank the size you described.
As far as land substrate goes, there are pit falls to each, if your doing half land half water, you need something that wont clog the filter in the water area, and something that wont color the water (bark chips may slowly dissolve in water).
Gravel is a good choice, but, if the frogs get some into their mouths when they are eating, it could cause problems.
Sand, I will not recommend, its usually a poor choice for amphibian substrates.
If you can find a large aquarium gravel that is too large for them to easily pick up and swallow when hunting thats what I would recommend.
When I had them, I used a 20gallon tank with a divider about 3" tall that was set in place with silicone and used it to divide the water/land areas.
I used a large washed river rock for the substrate in the dry area, and normal size aquarium gravel (or blasting sand) in the aquatic area.
I used a Duetto DJ50 (either a 50 or 100 cant remember) for filtration, and I even put a small scindapsus (pothos) ivy on the dry side buried down into the gravel.
They did quite well, and lived for several years on crickets.
Hope that helps.
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