Here's a basic Minecart dispenser that i designed. It holds 8 minecarts but you can make it hold as many as you like, all you have to do it make it taller. Basically you push the button and one Minecart drops from the pile, if the dispenser is full and you put in another Minecart it will just be redirected to a cactus. As i said this is just a basic one, and i'm sure if i carry on working with it i can improve it, such as adding an indicator to show when it is full. The only part of this design i copied from someone else is the trapdoor part to drop the carts which was from TaviRider, the rest I did my self (not saying that no one else has ever done one like this though).