You should only put around 2,500 URLs per sitemap in my opinion, since Google allows you to submit up to 500 sitemaps that should be utilizied. http://forums.digitalpoint.com/showthread.php?t=531765 is my sitemap FAQ you should review it.
i currently have 5000 , but i might increase it. currently google only hit the site every 1 minute. there was a day that it was hitting every 2 second.