How can I get the number of comments per page? I need the value "default comments per page" for special theming in comments-type.tpl.php for Drupal 6. Should I use variable_get()?
Those variables are stored per content type, named comment_default_per_page_TYPE. So to get the default comments per page for the article content type you would use:

$default = 50;
$per_page = variable_get('comment_default_per_page_article', $default);