It is not exactly like using the same password in multiple sites because you just need one remote site that stores your password in plain text or simple enough to reverse to compromise all the accounts that use the same password. For asymmetric encryption is the private part the one that should be safe (assuming the right algorithms) and just you should have access to it. If someone/something else can access/use them directly or indirectly then they may get compromised. And that may be a guide on why having multiple certificates, because you can discriminate with passphrases against i.e. local agents.
It is not exactly like using the same password in multiple sites because you just need one remote site that stores your password in plain text or simple enough to reverse to compromise all the accounts that use the same password. For asymmetric encryption is the private part the one that should be safe (assuming the right algorithms) and just you should have access to it. If someone/something else can access/use them directly or indirectly then they may get compromised. And that may be a guide on why having multiple certificates, because you can discriminate with passphrases against i.e. local agents.
I like using short-lived SSH certificates, and avoiding long-lived keypairs entirely.
Vendor post but it summarises the operational and security problems well.
https://smallstep.com/blog/use-ssh-certificates/
I prefer three: one for particularly important projects, one for normal projects, and one where security isn't a major concern.