Why resize sitemaps
Over 50,000 URLs, a sitemap must be split. Smaller files also make it easier to see which section has indexing problems in Search Console. Merging is useful when old tooling left several overlapping sitemaps.
Questions
What size should each part be?
Anything up to 50,000 URLs. Many sites use 10,000 or fewer per file.
Are duplicates removed when merging?
Yes, a URL listed twice is kept once, with its newest lastmod.
What is the limit for one sitemap?
50,000 URLs and 50 MB uncompressed. Larger sites split into several files listed in a sitemap index.
Is my data stored?
No. Sitemaps are read or built for the result you see and not kept.