We will notify you when you have used 80% of your credits for a given month. When you have used 80% of your monthly credits of a subscription, you will be notified via email. Once you have used up all credits, no further background removals are possible until next month (except for preview images on remove.bg which are always free). Should you need more credits, you can either 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.