You can enter up to 15 different dates on an application, and the application cost is $6.
But you can enter as many applications as your wallet can bear. When the lottery is run, as soon as any one of your applications is successful (awarded a permit slot), all the others are canceled.
By submitting multiple entries, you have a better chance of one of them being picked early in the process, so it will have a better chance of being successful. Most applications picked late in the process will likely not be successful due to all the slots being full for the dates on the application.
