The drupal default "Recent blog posts" only have 10 items.But I want show more.

Modify this file modules/blog/blog.module

function blog_block($op = 'list', $delta = 0) {
...
$result = db_query_range(db_rewrite_sql("SELECT n.nid, n.title, n.created FROM {node} n WHERE n.type = 'blog' AND n.status = 1 ORDER BY n.created DESC"), 0, 10);
.... }
}
}
}

Change that

ORDER BY n.created DESC"), 0, 10);

to

ORDER BY n.created DESC"), 0, 20);