Adding a user count within a shortcode

You may be able to do this by wrapping the shortcode in one of your own which would get the user count, insert it into the shortcode text, and do_shortcode() on the resulting text.

The sample function below should get you started. If you are not already using a Child theme, you should create one, otherwise you changes will be lost if you update your theme.

Add this to your child theme’s functions.php:

function my_statBox1_function( $atts, $content = '' ) {
   global $wpdb;
   $a = shortcode_atts( array(
         'stat_image' => 'http://www.dollarkiwi.com/wp-content/uploads/2014/08/man-icon.png',
         'stat_percentage' => 0,
         'text_color' => 'white',
         'class' => 'wow fadeInUp',
   ), $atts );
   $a['stat_percentage'] = $wpdb->get_var( "SELECT COUNT(*) FROM $wpdb->users" );
   $text = "[statBox1 stat_image=\"{$a['stat_image']}\" stat_percentage=\"{$a['stat_percentage']}\" text_color=\"{$a['text_color']}\" class=\"{$a['class']}\"] $content [/statBox1]";
   //return $text;  // For testing
   $result = do_shortcode($text);
   return $result;
}
add_shortcode( 'my_statBox1', 'my_statBox1_function' );

Then replace the statBox1 shortcode with my_statBox1 in your post.