There are so many recipes out there for Cupcakes, some use butter, some use oil, some even use only eggs and no fats.
These recipes will also call for an amount of ingredients that will yield so many cupcakes, some will give you 12, some 18, some 24 and so on. A recipe that makes 12 cupcakes for example, could give you a one layer 8 inch cake, but I am only speculating. Don't forget that Cupcakes take no more than 20 minutes baking, whereas a cake can take from 45 to even over 1 hour in the oven. You can always try and bake the batter in a single tin to see how tall it turns out.
*Edited to add: You mention you would like to make a rainbow cake. From what I've read, you are supposed to bake each layer independently, and they are thinner than a regular one layer cake so it would take less time in the oven per layer.
Hope that helps