Is it possible to register/renew a domain name for more than 10 years?
Unfortunately, 10 years is the maximum amount of years (between the current date and the current expiration date) for which a domain name can be registered.
For example, a domain was registered on January 1, 2017 for 1 year, so its current expiration date is January 1, 2018, and let's say today it's June 1, 2017. At the moment you can renew your domain for 9 additional years maximum and its new expiration date would be January 1, 2027.
Now, let's imagine that you had renewed this domain and today, on June 1, 2022, you feel the need to extend the registration for more years. Still, you have 4 years and 5 months of your original registration left. Since the maximum registration period is 10 years it means that you can add only 5 years and 7 months to your domain.
However, the general rule is that domains can be extended only for full years, which makes it possible for you to add 5 years only. Following the same logic, you'll have a possibility to renew your domain for more years in future as well.