WordPress MU uses one database and each blog gets his own tables with the blog id in the prefix, so once you install a network installation your database should have these tables:
- wp_1_categories
- wp_1_comments
- wp_1_link2cat
- wp_1_links
- wp_1_options
- wp_1_post2cat
- wp_1_postmeta
- wp_1_posts
- wp_blogs
- wp_blog_versions
- wp_registration_log
- wp_signups
- wp_site
- wp_sitecategories
- wp_sitemeta
- wp_usermeta
- wp_users
And every new site you set up will create separate tables for example:
- wp_2_categories
- wp_2_comments
- wp_2_link2cat
- wp_2_links
- wp_2_options
- wp_2_post2cat
- wp_2_postmeta
- wp_2_posts