I don't know about Passkeys specifically, but this is unfortunately common enough with WebAuthn rollouts.

I'm not sure if it's true anymore, but Twitter for years only supported a single WebAuthn token.

I think even Amazon does that too still, such a shame

If you’re referring to AWS, they added support for multiple MFA devices last year:

https://aws.amazon.com/blogs/security/you-can-now-assign-mul...

Amazon’s shopping site also lets you set up multiple devices, but I’m not sure when they added that.

No Webauthn support at Amazon.com whatsoever. It's mandatory SMS (you must provide a number and you can't say "I don't want this to be even a backup option") with optional TOTP.

Tailscale only allows one passkey it seems.