I actually think they need 2, usually need a second domain / setup for failover. Especially if the primary domain is a novelty TLD like.. .IO which showed that things can happen at random to the TLD. If the website down it's fine, but if you have systems calling back to subdomains on that domain, you're out of luck. A good failover will help mitigate / minimize these issues. I'd also keep it on a separate registrar.

Domains are really cheap, I try to just pay for 5-10 year blocks (as many as I can), when I can just to reduce the issues.