Filled in customer number

Hello
Is it possible to do so that person / organization number must be filled in to be able to place orders?

You can either edit the view and insert the parameter required for the field.

<?php echo functions::form_draw_text_field('tax_id', true, 'required'); ?>

https://github.com/litecart/litecart/blob/a49ba67108ea78960a33888eed6f9657f588ce73/public_html/includes/templates/default.catalog/views/box_checkout_customer.inc.php#L26

Or create a customer module that returns an error in validate().

      if (empty($this->settings['status'])) return;

      if (empty($data['tax_id'])) {
        return ['error' => 'You must provide your CRN or company number'];
      }
    }

https://wiki.litecart.net/how_to_create_a_customer_module

Or both if you want to be secure.

Thanks