If you have only one pan and a recipe calls for more cupcakes than your pan will make, just 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.
An easy way to fill baking cups is to use an ice cream scoop. Use one that measures out about 1/4 cup batter if filling regular-size cups. Use one that measures out 1 heaping tablespoon batter if filling mini cups.