Can’t increase posts_per_page by variable

All right, I think I see what’s going on…

As per WP pagination parameters documentation, posts_per_page is indeed cached. Somewhat… Basically, you could set this to “10” the first time, as you do, and then to 5 after that, but then use “offset” or “paged” parameters to get the posts you want.

Your “load-next-amount” variable would simply be a counter of which “offset” or “page” you want to access…

Using offset, you could pass “load-next-amount” equals to “0”, “1”, “2”, …

$myArgs = array(
    'category'         => '',
    'category_name'    => '',
    'orderby'          => 'meta_value_num',
    'order'            => 'ASC',
    'include'          => '',
    'exclude'          => '',
    'meta_key'         => '_price_per_1hr_lesson',
    'meta_value'       => '',
    'post_type'        => 'job_listing',
    'post_mime_type'   => '',
    'post_parent'      => '',
    'post__in' => $what,
    'author'       => '',
    'author_name'      => '',
    'post_status'      => 'publish',
    'job_listing_region'  => "'" . $city . "'",
    'post_status'      => 'publish',
    'posts_per_page' => 10, // <--------
    'offset' => 10 + (intVal($_POST['load-next-amount']) * 5), // <--------
    'suppress_filters' => true 
);
$myquery = new WP_Query( $myArgs );

Hope this helps!