Sign arbitrary data with your SSH keys

So a little offtopic but I’m still curious: how do you handle multiple machines and SSH keys? I mean do you run ssh-keygen on a new machine and have for each computer a separate key pair or do you have one key pair that you copy on every new machine?

I have seen both and using one key pair looks very convenient but also makes me feel a little uneasy.

I myself have a key pair for each of my machines.

How do you handle it?

I use two keypairs, but with no ability to read the private key (smartcard).