prefix_check_minimum_uploads.php

<?php
/**
 * Custom validation to set minimum number of uploads
 */
function prefix_check_minimum_uploads( $passed, $post, $item ) {

	// Enter the Group ID for the upload field
	$group_id = 2908;

	// Enter the Field ID of the upload field
	$field_id = 2909;

	// Set the minimum number of uploads required
	$min_uploads = 3;
  
  // Set your message if the user doesn't upload enough files
  $message = 'Please upload 6 files';
  
	$field_name = 'pewc_group_' . $group_id . '_' . $field_id . '_number_uploads';
	if( isset( $post[$field_name] ) && floatval( $post[$field_name] ) < $min_uploads ) {
		wc_add_notice( $message, 'error' );
		$passed = false;
	}
  
  return $passed;
  
}
add_filter( 'pewc_filter_validate_cart_item_status', 'prefix_check_minimum_uploads', 10, 3 );