> Do you use the same SSH keys on multiple devices?

Assuming you mean client devices, yes, depending on my personal relationship/control of the device. (For servers, the answer is "very yes".)

For example, my personal laptop and desktop may have the same private key, and I will backup/restore that same key onto either of them if they are reinstalled or replaced with new hardware.

However my work laptop gets its own, so that I can more-easily limit what it can access or cancel it in the future.