It needs to be updated if you add more pages on your website. You don't need to resubmit your sitemap, that's nonsense.
If you are using Wordpress and using a sitemap generator tool, then the sitemap should be automatically updated when you publish any new post. You do not have to do anything extra for that. Moreover it will also notify all the major search engines automatically. If you ask me which one is the best for Wordpress, I would suggest to use Google sitemap generator plugin.
If you add new link to Your website You should also put it into sitemap, if You use Wordpress You can get a plugin which will be doing it for You