tthey will roll over unless you cancel your account. As long as your account is live they will roll over to a Max of 120 credits.
From the FAQ: Adobe Stock Help | Common Questions
What happens if I don’t use all my monthly licenses in a given month?
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.