Dividing a Sitemap

350+ links and growing

[contentmaster]

Our website's sitemap now contains about 350 links and hasn't been updated for quite some time.

Is it correct that a sitemap should not contain more than 100 links? If yes, what is the best way of making it more manageable, now that it has 350+ links?

How often should you update your .htm sitemap? Every time a new webpage is added to the website?