No, InDesign isn't a one time purchase; after the initial purchase of your version (cs6, cc, etc.), you're able to upgrade to the next version or purchase a subscription for Creative Cloud.
If you choose to buy a one-time purchase of InDesign CS6, you'll get all the features of InDesign with a one-time fee; however, you won't be able to get updates or upgrades to the product.
On the other hand, subscribing to Adobe Creative Cloud is a monthly fee; you have access to all of their creative suite apps and regular updates & new features. After your subscription ends, you'll keep access to the apps and all your files, but you won't get the updates or newer versions beyond the most recent version of the app you subscribed to.
Ultimately, it boils down to whether you want to pay a one-time fee for the version you buy or pay a monthly fee for access to a suite of apps and updates.
For more information, you can check out the following article:
https://helpx.adobe.com/au/creative-cloud/help/buy-creative-cloud.html