I'd be more concerned about general reliability. How long will these last? Because I think they should last at a minimum 10 years. If you only use one for an account, and it breaks on you, you're screwed.

So I would use at least 2, but hopefully websites will allow this, and that will probably be the largest bottleneck in the future. I couldn't care less about "SMS backups" or such nonsense, as that completely defeats the point of using a hardware token. Your account will only be as secure as your phone number is.