As you can see, you only need one of these to go into the cactus nursery business. This is a very easy cactus to grow. If you put it in a small pot and don't feed it, it will just sit there and look pretty for years on end. If you put it in a large pot and feed it, it will grow. It's more tolerant of water than most cactus.
The pups can be knocked off and planted at any time. Even the very small ones will grow with very few losses. If you keep all the pups trimmed off the main plant will grow as large as 6 inches across and almost as tall; if it gets much larger it tends to start growing unevenly.
Gymnocalicium damsii can be grown in full shade, although growth will be very slow, if it grows at all. Just a couple of hours of morning sun will do, although you'll want to rotate it periodically so it won't be lopsided. If you grow it on a windowsill it will get lopsided no matter how much sun you give it. I keep mine outside in full sun with regular watering in the summer.
In the colder months, I stop watering most of my cactus (I only have about three 'colder months' in south Texas). In cactus soil with no water, Gymnocalicium damsii will sometimes get alarmingly shriveled and yellow, but in the springtime with lots of water it will fill and green again with no problem. I've left these guys bare-rooted outside on a shelf for up to six months with no problem other than the occasional mealy bug infestation, which is easy to deal with.
This is one of my 'filler plants'. I'll put pups into pots with other cactus that would otherwise look too small for their pots, because they are generally slow-growing and tolerant of such a wide range of conditions, and because I have so freakin' many.