The use of HikaShop is not limited to one website, however, the auto update service that we provide with your purchase is only for one website, like the other services (cron task, access to updates, support, ...).
Note that you can simply download the package on our website via the menu Your account>Your subscriptions and do the update manually on all the websites. You can also manually change the URL of the subscription, purge the cache of your updater and then do the automatic update on the website.