[Muromec]

Why would I want to have one key for all them? To lose access or get them all compromised at the same time?

[nmca]

I think your threat model is bad — the isolation of accounts offers little and most people have effectively only the security of their email provider anyway.

That said, buying several keys would be a natural and happy path solution for the paranoid. Make sure to get three for each account (same as primary) as either you need a backup or the extra key offers no extra security.