So when we have a website that contains 50,000 + URLs [one sitemap is only allowed to have 50,000 or less URLs] or the file-size of an individual sitemap increases from the allowed size of 10MB [Uncompressed Size], then we use sitemap index file [SIF] to include multiple sitemaps.
A sitemap index file is basically a list that contains URLs to different sitemaps. SIF do not have URLs of a website: instead: it contains URLs of multiple sitemaps.
So sitemap index file is the solution for those people or sites: where there is a need of multiple sitemaps: but how many sitemap URLs can we include in a single Sitemap Index File?
SIF also have a limitation on the number of sitemap URLs that we can put in it.
A SIF cannot have more than 1,000 [One Thousand] URLs of sitemaps.
Now you may have a question in your mind: What if you reach this limit for SIF? That is: what if you need to add more than 1,000 URLs in a SIF?
First let’s see how many URLs can be included in one SIF:
1,000 x 50,000 = 50,000,000.000 URLs
So one SIF can point to different sitemaps which can contain up to 50 Million URLs. Does your site have more than 50M URLs? : )
Anyway: I guess the simple solution is to have more than one sitemap index file: incase the SIF reaches its limitations.