Wordpress - How to get the number of Pages in a single Post Pagination?

have a look at https://codex.wordpress.org/Global_Variables#Inside_the_Loop_variables

within the loop, the global variables related to the <!--nextpage--> tag are:

$page - the actual sub page,

$multipage - (boolean) =1 if nextpage was used

$numpages - total number of sub pages

Inside the loop, as michael pointed out you could just refer to $numpages global variable.

Outside of the loop, your code is almost fine.

Being the number of the pages the number of <!--nextpage--> + 1, you could save a function call by doing:

$numOfPages = 1 + substr_count($the_post->post_content, '<!--nextpage-->');

As a quick and not-that-dirty solution that is totally fine.

However, that code would not take into account filters and other minor things (e.g. <!--nextpage--> at the very start of the post content is ignored).

So to be 100% compatible with core code, your function should be something like:

function count_post_pages($post_id) {
    $post = get_post($post_id);
    if (!$post) {
       return -1;
    }

    global $numpages;
    $q = new WP_Query();
    $q->setup_postdata($post);

    $count = $numpages;

    $q->reset_postdata();

    return $count;
}

This is 100% compatible with core code, but also is more "expensive" and triggers more hooks than you actually need (with possibly unexpected side effects).

The third solution would be:

function count_post_pages($post_id) {
    $post = get_post($post_id);
    if (!$post) {
       return -1;
    }

    $content = ltrim($post->post_content);
    // Ignore nextpage at the beginning of the content
    if (strpos($content, '<!--nextpage-->') === 0) {
         $content = substr($content, 15);
    }

    $pages = explode('<!--nextpage-->', $content);
    $pages = apply_filters('content_pagination', $pages, $post);

    return count($pages);
}

This solution is relatively "cheap" and it is 100% core compliant... for now, in fact, the issue with this code is that it duplicates some code that might change in future versions (e.g. the "content_pagination" filter was added in version 4.4, so quite recently).