WordPress | ACF | Selected or fallback

selected_or_fallback.php

<?php
/**
* Works with the ACF relationship field to always display the quota of posts
*/
function selected_or_fallback( $current_id, $total_required, $acf_relationship_name, $post_type = null) {
    $excluded = array( $current_id );
    $obtained = 0;
    $related = array();

    // Check to see if any posts have been selected via ACF relationship
    $selected = get_field( $acf_relationship_name );
    if( $selected ) {
        $related = $selected;
        $obtained = count( $selected );
        if ( $obtained < $total_required ) {
            // If the quota not met update excluded array for get_posts query
            $selected_ids = get_field($acf_relationship_name, false, false); // Returns an array of IDs
            $excluded = array_merge($selected_ids, $excluded);
        }
    }

    if ( $obtained < $total_required ) {
        if( $selected ) {
            $post_type = get_post_type( $selected_ids[0] );
        } else {
            if ( !$post_type ) {
                $post_type = get_post_type( $current_id );
            }
        }
        $required = $total_required - $obtained;
        $args = array(
            'posts_per_page' => $required,
            'post_type' => $post_type,
            'exclude' => $excluded
        );
        $posts = get_posts( $args );
        if( $posts ) {
            $related = array_merge($related, $posts);
        }
    }
    return $related;
}

Update October 2020

Further update allowing you to pass in a post-type variable. Whilst the last update was good, it failed where no posts were selected on somewhere like the homepage as the function wouldn't be able to determine the intended post-type and would instead display pages.