Males are territorial, so really only one should be kept in each enclosure. Ideally, bullfrogs should be given a container appropriate to their size, I don't recommend anything smaller than a 75g tank or equivalent. If I were keeping them at home I certainly would make a custom enclosure that maximized floor space. Contrary to what most people say, I don't think that bullfrogs need much water as adults. All of the bullfrogs I use on exhibit rarely enter the water part of the tank, especially when provided with a water dish to sit in.
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...the oldest task in human history: to live on a piece of land without spoiling it."
Aldo Leopold (1938)
"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)