In my sitution, all users can create blogs, but considering the performance and speed, we need to limit the number of creating blogs.For example, part of users (as students) can only create one blog (someone may never create blog), part of users (as teachers) can create 2-3 blogs, and part of the user can create more. So it's very useful if we can limit the nums by user group. I found a way to do so, but it's a pity that it's a hack but not a plugin. So if someone can make it as a plugin would be much better.

1.At inc\collections\_collections.init.php lines: 449 ,add 'perm_createblog_nums'=>0, so it like this:

return $permissions = array( 'perm_api' => $permapi, 'perm_createblog' => $permcreateblog, 'perm_getblog' => $permgetblog ,'perm_createblog_nums'=>0);

At the same time , lines: 458, in function get_available_group_permissions(), add an array:

'perm_createblog_nums' => array(
				'label' => T_( 'Allow creating blog numbers'),
				'user_func'  => 'check_createblog_user_perm',
				'group_func' => 'check_createblog_group_perm',
				'perm_block' => 'blogging',
				'perm_type' => 'text_input',
                                'maxlength' => 2,
				'note' => T_( 'It takes effect when “Users can create new blogs for themselves” has been checked'),
				),

2. In inc\collections\collections.ctrl.php, lines 59, add

$current_User_PermCreateBlogNums = $current_User->get_Group()->get_GroupSettings()->get("perm_createblog_nums",$current_User->get("grp_ID"));      
                if($current_User_PermCreateBlogNums <= $current_User->get_num_blogs())
                {
                        $Messages->add( sprintf( T_('The number of blogs you have created is: "% s" has reached the limit.The upper limit is: "% s". '),$current_User->get_num_blogs(),$current_User_PermCreateBlogNums   ) , 'error' );
                        $redirect_to = param( 'redirect_to', 'url', $admin_url );
			header_redirect( $redirect_to );
                }