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