From the FAQ: Common Questions, Adobe Stock
With a small subscription plan, which includes 10 images a month, unused downloads carry over to the next month as long as your subscription is active. The carryover limit is 120 downloads. With large subscriptions, which include 750 downloads per month, unused downloads do not roll over.
Rollover also applies to the 3 standard assets/month plan - up to 36, and the 40 standard assets/month plan - up to 480.
Hope this helps.