You can learn the basics about the wp_list_categories() function from the WordPress Codex.
By default this function will not show up the post count, but if you will activate it by show_count=1, the post count will be surrounded by parentheses. To remove the parentheses without modifying the WordPress core files, you need to use the following code:
<?php
$variable = wp_list_categories('echo=0&show_count=1&include=1,2,3,4,5&title_li=');
$variable = str_replace(array('(',')'), '', $variable);
echo $variable;
?>
