We will notify you when you have used 80% of your credits for a given month. When you have used up all credits, no further API calls and high-resolution background removals are possible and you are restricted to free actions (i.e. small image downloads on our website) until next month. If you need more credits, you can upgrade your plan or purchase a pay as you go package.
If you want to make sure you never run out of credits, have a look at this article.