Usually people will just create one key with just default command
get a public/private pair in (/Users/my_user/.ssh/id_rsa) and use that for anything, but there is much more to this then that. You can create many keys and use them for separate accounts and also automate that.
ssh-keygen -f ~/.ssh/<username> -C "firstname.lastname@example.org"
this will generate key with specific username and email you want and save it as such. Then you can add this info to config in ~/.ssh/config
and use it from there to connect which ssh key to use with which host like
Host github-org User git HostName github.com IdentityFile ~/.ssh/github.org.key
so you can have a key for each of the accounts/hosts you are using for some additional security and separation.
For some more info on that look here
Before finishing all, you need to add this new key to SSH agent, you do that with
ssh-add -K ~/.ssh/new_key
On mac OSx if you reset your computer, you will need to add it again. Other wise you will get
Permission denied (publickey) error. To have that autoloaded update your ~/.ssh/config to this
Host github-org User git HostName github.com AddKeysToAgent yes UseKeychain yes IdentityFile ~/.ssh/github.org.key
If you're using macOS Sierra 10.12.2 or later, you will need to modify your
~/.ssh/configfile to automatically load keys into the ssh-agent and store passphrases in your keychain.
Some tips on using multiple keys and how to make it work in easy way