For sitemaps such as url lists and xml, you can put 50000 urls per page of the sitemap, more than that should be split and an index made.
But I think that an html sitemap, as Bagi Zoltan said, being html pages, Google does reccomend 100 links per page, which generally refers to normal site pages, but as an html sitemap is more for visitors use, it may still apply.
I use the sitemap generator provided at
Create your Google Sitemap Online - XML Sitemaps Generator for a large site. I find it good, it creates xml, html, ror and url lists and you can specify how many urls per page for the html sitemaps and xml as well. So you could split a large sites urls over several pages in the html sitemaps using that.