The method for increment you used will not achieve what you want to if you delete the posts. Instead you can save the counter in wp_options table on front-end form submission, something like:
if(get_option('customers_count')){
$count = get_option('customers_count', true);
update_option('customers_count', $count+1);
} else {
/** This will automatically add the option if it does not exist. **/
update_option('customers_count', 1); // adding first time as value 1
}
It will always take new number as post title #.
Note: Make sure you update option only when submit form for adding a new post and not for edit post.