Use the below snippet to add map markers from custom post types.
This example uses a repeater field to allow for multiple markers from one post, use a regular field to for just one marker.
<?php
$args = array(
'post_type' => 'cpt-name',
'posts_per_page' => -1
);
$the_query = new WP_Query( $args );
?>
<div class="acf-map">
<?php if ( $the_query->have_posts() ) : while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
<?php while ( have_rows('countries') ) : the_row();
$location = get_sub_field('country');
?>
<div class="marker" data-lat="<?php echo $location['lat']; ?>" data-lng="<?php echo $location['lng']; ?>">
<p><?php the_title(); ?></p>
</div>
<?php endwhile; ?>
<?php endwhile; endif; ?>
</div>