Best Practice: ”separate ssh-key per host and user“ vs. ”one ssh-key for all hosts“

Best Practice: ”separate ssh-key per host and user“ vs. ”one ssh-key for all hosts“

Is it better to create a separate SSH key for each host and user or just using the id_rsa key for all hosts to authenticate? Could one id_rsa be malpractice for the privacy/anonymity policies?

having one ssh-key for all hosts:

~/.ssh/id_rsa
~/.ssh/id_rsa.pub

in comparison to separate ssh-keys:

~/.ssh/user1_host1
~/.ssh/user1_host1.pub
~/.ssh/user2_host1
~/.ssh/user2_host1.pub
~/.ssh/user3_host2
~/.ssh/user3_host2.pub
~/.ssh/user4_host3
~/.ssh/user4_host3.pub
... etc.