XML sitemaps help the search engine find deep links to your website. If you have links to every page on your
site (internal or external links) then you don't need a xml sitemap, but its good practise to add one. You can also
use it to tell the search engines how often to visit per page. If your site is big like a forum, I would say
its essential to get a sitemap. There are plenty of mods you can get. If you use wordpress, they have a xml sitemap generator.
Every time you do a major change to your site, just as adding pages, please resubmit your sitemap to google in
webmaster tools. You should be updating your site at least once a month.
Maximum Size Sitemap.
You can have 50,000 urls per page, and 50,000 indexes of those pages. Which means that you can have up to 2.5
billion pages in your sitemap.
How can I add more than 1,000,000 URLs to a Sitemap?
How do I get more webpages indexed?
Sitemap for Wordpress Users.
If you are using wordpress, you can use the xml sitemap plugin and it will automatically updage your sitemap and ping google, everytime
it changes.