Usually after 500 pages you will have to pay for the generation of sitemaps, however i am unsure whether there is any free tool for doing 500 pages or more. The only one i use is xml-sitemaps.com however as i said thats limited to 500 pages.
Try www.freesitemapgenerator.com, they have a free and a paid service. The free service is slower as you have to wait in qeue.