"How many babies do Chubby Dragons have?"
Chubby Dragons generally have one baby. At rare times, the Ethereals might find a pairing perfect enough for twins. If this is the case, then two babies are born, and not one. At even rarer times, the Ethereals grant the parents triplets. However, they need to take one of the babies for their own. Which they take is the parent's choice, but this baby will become an Ethral Runner.
"If I got Chubby Dragons, how would I breed them?"
Every full moon, SPO takes it upon herself to open the Breeding Niche to the public. At this time, she will post a thread on PonyIsland. There, you will find a form to use and you will see that the Valley General is opened. That is when you may purchase items and redeem Breeding Vouchers for items.