Let's say you don't use all downloads in this month and your monthly-subscription is still active. How many months will the unused downloads carry over?

All Unused downloads carry over each month, for up to 3 month maximum.

Means if you have i.e. 20 downloads left in January - you can still use them in March. The oldest unused downloads will then expire. 

Keep in mind, that your monthly subscription MUST BE ACTIVE to have the unused downloads carry over. If you cancel your monthly subscriptions your unused downloads will expire.