I use Demo Data Creator plugin with it you can create and set:
- Number of users to create
- Number of blogs per user (for WPMU/MultiSite)
- Whether users must have a blog
- Number of categories in each blog
- Number of posts in each blog
- Number of paragraphs in each blog post
- Number of pages in each blog
- Number of top-level pages
- Number of levels to nest pages
- Number of comments per post for each blog
- Number of links in blogroll for each blog