How to give titles to custom post type as “unique” incremental number?

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.