You don’t need that outside acf/init action to do this.
Try this in your functions.php file (I changed the function name):
function acf_prepare_field_set_print_max( $field ) {
$project_total_prints = get_field('project_total_prints');
$field['max'] = $project_total_prints;
return $field;
}
add_filter('acf/prepare_field/name=print_nbr', 'acf_prepare_field_set_print_max');