Not every URL crawled in Site Audit spends your Crawl Credits.
Only internal HTML pages that return the 200 (OK) HTTP status code consume your monthly Crawl Credits as they're crawled.
The following types of URLs won't spend your credits when they are crawled:
3xx redirecting URLs;
4xx and 5xx URLs;
Resources (images, CSS files, and JS files);
Any external URLs linked from your website.
Depending on your subscription plan, Ahrefs' Site Audit can analyze from 100K to 5M pages per month on your websites.
For each verified project, the first 5,000 crawl credits per month spent to crawl this project is free. This is available for users on all plans.
You can limit the maximum number of internal pages per crawl. Correspondingly, this will limit the maximum number of credits that can be spent during the crawl.
To do that, go to the Project settings and specify the "Max number of internal pages."
Once the crawl is finished, you will most likely see that the "Total URLs crawled" for your project exceeds this limit. This happens because the URLs other than internal 200 (OK) HTML pages are crawled at no cost and are not limited.