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The Google index lists all the webpages that Google knows about. It contains hundreds of billions of webpages and is constantly updated with new pages. To appear in search results, a web page must be in the Google Index.
To be indexed, a page needs first to be crawled. Google assigns a crawl budget to every website. The crawl budget is defined as "the number of URLs Googlebot can and wants to crawl". If a website has too many pages compared to its crawl budget, not all of its pages will be crawled and indexed by Google.