The answer is no.
If you are subscribed to Pro Micro or Pro Starter and you did not use all available downloads, these will not be rolled-over to the following month. The unused downloads expire and you will get 10 brand new downloads under Pro Micro plan or 50 new downloads under Pro Starter plan for the following month of subscription.
If you are not using all your available downloads in one month, you can always downgrade your plan to another one that fits your icon needs more. Read about How to change your subscription.
If a subscription plan is not suitable for you, you can switch to the Pay-as-you-go option and return to subscription once you are in need of icons again. Check out all the available options on the Buy icons page.