This is one of those areas that really depends on the details and on your knowledge as an aquarist. True pygmies (maximum mantle length under 3-4 cm) can do well in the right kind of 20 gal system, but it has to be single species, be well run in and rock stable, and hopefully have a small protein skimmer in case of an inking event. You also want to have a good supply of sea water in case of an emergency. We keep about 20 tanks, all under 20 gal, with a variety of pygmies such as O. wolfi, O. micropyrus, O. mercatoris, O. bocki , juvenile bi-macs and blue-rings. With frequent water changes the animals do well. However, these are research tanks that are pretty plain and bare-bones. They don't look good, but they keep the animals alive.
Roy