Counting how many posts are filed under one tag

Hi!

On the archive page, I want to display how many posts are covered by the relevant category / tag archive.

The following code shows the number of posts of a category:

<div class="archive_number">
This category contains <?php $cat = get_the_category(); $cat = $cat[0]; echo $cat->category_count; ?> posts
</div>

Now, I want to display the number of posts filed under a tag. I’ve tried it with the following code:

<div class="archive_number">
<?php $tag = get_query_var('tag_id'); $tag = $tag[0]; echo $tag->tag_count; ?> posts are filed under this tag.
</div>

But (of course) this does not work. Any ideas what I do wrong? Be patient, I’m a php n00b.

Thanks for your help.