It depends on the subscription your purchased. Obviously, you can purchase the Lightroom Plan or the Photography Plan from within a browser on a tablet too, but if you purchase the 'Premium' subscription from within Lightroom mobile, then you buy a subscription that is only for Lightroom mobile.
Are you wanting to use Lightroom Classic? You posted this question in the forum for Classic. If you want to use either Lightroom, or Lightroom Classic on a desktop computer you need a subscription from Adobe. https://www.adobe.com/creativecloud/plans.html