By default, the ssh client will try all keys it found in the ~/.ssh directory.
So if you have a lot of them, it can be problematic because it will failed a lot of time and eventually be kicked by the SSH server with this error:
Too Many Authentication Failures
To avoid this behaviour, add the following option to the command line (or directly in the ssh configuration file)
And then specify the method you want to use
ssh -o IdentitiesOnly=yes -o PreferredAuthentication=pubkey -i ~/.ssh/my_new_key root@<ip>