You have reached BigCommerce API calls limit
Standard and Plus plans have a limit of 20,000 API calls per hour, unlike Pro plan, that features 60,000 API calls per hour. As for Enterprise clients, they have unlimited API calls. In case you hit this threshold, don’t worry, your migration is still ongoing. All your Product Variants will be transferred accurately and securely, but it will take a little bit longer. These limits were submitted by BigCommerce, that’s why Cart2Cart cannot exceed them. Just chill out and wait, the replatforming process will be completed as soon as possible.