You could use a relationship field instead. A post object field that allows multiple values is just another way of displaying a relationship field.
Or you can create an acf/validate_value filter https://www.advancedcustomfields.com/resources/acf-validate_value/. $value in this case will be an array of post IDs and you can do
if (count($value) > $my_max_posts) {
$valid = 'No more than '.$my_max_posts.' allowed';
}
I’d also add a note to the field instructions about the maximum.
You could probably also do this using javascript and the select2_init hook to alter the select2 settings https://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/
