I have multiple SSH keys – I prefer to use a unique one for each client. However, when you add a key to the SSH client with

it only persists for as long as the client is running if you didn’t add the SSH key to Keychain when you first used it – if you reboot frequently (which I don’t normally, but since I bootcamped, I do more often) that key is lost and needs to be added each time.

Adding it to the keychain at the same time is as simple as using the -K switch:

Usually keys are stored in the .ssh directory for your user

assuming the key is named id_rsa_mykey, which is the format I use. Note it is the private keyfile you add, not the public one (id_rsa_mykey.pub)