inserting a variable's value into a post

The ways I can think of to do this would be a plugin that allows php code to be written into the posts.

There are some, but none I can see that look to have been updated for over a year.

The other is to create a function of your own in functions.php which will allow you to add a shortcode to the posts.

Then use wp_count_posts()

Untested Function WARNING:
This code should not be tested on a production website only test on a local install.

function my_post_count() {
	$count_posts = wp_count_posts();
	$published_posts = $count_posts->publish;

	return 'we have ' .$published_posts. 'posts as of today';
}
add_shortcode('post_count', 'my_post_count');
add_filter('my_post_count', 'do_shortcode');
/* End Shortcode Post Count [post_count] */

In the post you would use a shortcode

Did you know that [post_count], why not check out some of the others?

HTH

David