When I buy a license for a year for the events calendar pro plugin.
Do I get only one year updates?
So do I need to buy every year a license of 89 dollar?
Yes, that is correct: a license provides with with one year of access to plugin updates and support. You can renew once the license expires, or keep using the plugin at its current version.
Does this help answer your question? Please let me know.
One other question. Is there a garantee that I can use the plugin after finished the license has expired? Or do I need the updates when I update WordPress?
The plugins should continue to function as they normally would, but the chances of compatibility with WordPress (or any other theme and plugin updates) do certainly start to decrease the further along WordPress develops. Also, I would recommend not updating The Events Calendar once the Events Calendar PRO license expires because those two plugins rely on one another and will be less likely to function together as updated versions are released. Best result would be to keep those at the same version.