well, you may have a look at my plugin i recently created.It will add an option to SiteAdmin->options page ,where you can limit number of blogs per user.
Here is the code ,which does it.
add_filter("wpmu_active_signup","tiw_check_current_users_blog"); //send fake/true enable or disabled request
add_action("wpmu_options","tiw_display_options_form"); //show the form to allow how many number of blogs per user
add_action("update_wpmu_options","tiw_save_num_allowed_blogs");//action to save number of allowed blogs per user
/****Check ,whether blog registration is allowed,and how many blogs per logged in user is allowed */
if( !is_user_logged_in() )
return $active_signup;//if the user is not logged in,do not change the site policies
$blogs=get_blogs_of_user($current_user->ID);//get all blogs of user
//if number of allowed blog is greater than 0 and current user owns less number of blogs */
/****How many blogs are allowed per user *************/
$num_allowed_blog=get_site_option("tiw_allowed_blogs_per_user");//find how many blogs are allowed
return $num_allowed_blog;//return the number of allowed blogs
/*****Show the Number of Blogs to restrict per user at the bottom of Site options ****/
<h3><?php _e('Limit Blog Registrations Per User') ?></h3>
<th scope="row">Number of blogs allowed per User</th>
<input type="text" name="num_allowed_blogs" value="<?php echo tiw_num_allowed_blogs()?>" />
<p>If the Value is Zero,It indicates any number of blog is allowed</p>
/**************Save the Number of blogs per user when the form is updated **************/
$allowed_number_of_blogs=intval($_POST["num_allowed_blogs"]);//how many blogs the user has set
//save to the database
or if you wish to have the plugin you may have a look here http://www.thinkinginwordpress.com/2009/03/limit-number-of-blogs-per-user-for-wordpress-mu-and-buddypress-websiteblog-network/
Sorry,admins,if you think,linking my page is against rules, please remove that,as I have already posted the code here.