It depends on whether you buy a perpetual (one-time) or annual (currently only available for SaaS and OEM) license.
For a perpetual license, you pay a one-time fee and receive access to the current major version of ZingChart (currently 2.x.x). That means you will receive access to all of the new features, updates, and bug fixes associated with that version, which are generally released every month or two as either a minor (feature) release or patch (bug fix) release. When ZingChart upgrades to the next major version (3.x.x), perpetual license holders would need to purchase an upgraded license if they want access to that version. Read the page for your particular license for more details.
For an annual license (currently only available on SaaS and OEM), you will always receive access to the current major version of ZingChart, so long as your annual fee is paid. For more information on the annual SaaS license, read this page. For more information on the annual OEM license, contact our sales team at firstname.lastname@example.org.