Under current rules, domain names can be registered for up to 10 years into the future. That doesn’t mean you can only own the domain for 10 years, that just means that you can register for up to 10 years into the future at any given time. You can register for 10 years, then wait a year so it has 9 years left, then renew it for another year and keep it at 10 years.
Some registrars try selling longer registrations, but all they are really doing is getting your money up front and not actually registering the domain for as long as you pay them for. If they go out of business or you transfer your domain to another registrar you’d lose the extra time.