It depends on how your site is constructed and what your sitemap generator software is.
If you have a Wordpress or other CMS with plugins that create an XML sitemap, they can automatically generate a new sitemap everyday so you are sure that you get to submit to the search engines your new content every day
Otherwise, it is best to regenerate and resubmit your sitemap if you add new pages so Google and other SEs can easily crawl your new pages.
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