If you have only one pan and a recipe calls for more cupcakes than your pan will make, cover and refrigerate the rest of the batter while baking the first batch. Cool the pan about 15 minutes, then bake the rest of the batter, adding 1 to 2 minutes to the bake time.
Check your market for seasonal fresh Key limes in bags, and juice them. Key limes are smaller than regular Persian limes, about the size of a golf ball. You will need 5 Key limes to make about 1/3 cup juice.