With the Pro Starter plan, you get 50 new downloads every month. These downloads are not aggregated, meaning that if you do not use all of your downloads in the current month, they will not be added to the next month's total. Instead, you will start with 50 new downloads.
For example, if you use 25 out of 50 available downloads in May, the remaining 25 downloads will not be added to June's total. In June, you will still start with 50 new downloads.
The subscription plan is an auto-renewing plan. This means that you will be automatically charged every billing period (either every month or year depending on the billing option you have chosen). To stop your subscription, you need to cancel it before the next billing period. Read about How to cancel your subscription.
A subscription plan can be cancelled anytime. It can also be reopened after the cancellation. If you only need it for 1 month, do not forget to cancel it before the next billing period is due. You can read about it in this article: How to subscribe to Pro for only 1 month.