They won't require a genetically stable population in each because they will be close enough to travel between each other. So, if we postulate a biosphere size of the largest stadiums on Earth (100,000), Earth's distance around the sun (forgive me for using miles) is 584, 088, 920 divided by 40,000 = 14, 600 miles between biospheres. So about a long trip by plane on Earth. There are But the number of miles between biospheres can be raised and therefore the number of biospheres lowered depending on how fast the shuttle craft between biospheres can travel. I'm assuming they will travel faster than a jet.