I would probably stand up an LDAP server after I had 4 or 5 user accounts to deal with, or, more importantly, more than, say, 3 servers to deal with.
I think there's a lot of fear and hate surrounding LDAP, but that's mostly for historical reasons. LDAP has gotten a lot easier to set up. Even in 2009 a colleague and I set one up (using openldap) and had other machines authenticating off of it in an afternoon. It's gotten even easier than that since then.
And hell, you should be using Chef to set up your LDAP master and slave. So once you have the config correct once, you can bring up another machine without trouble when needed.
I think there's a lot of fear and hate surrounding LDAP, but that's mostly for historical reasons. LDAP has gotten a lot easier to set up. Even in 2009 a colleague and I set one up (using openldap) and had other machines authenticating off of it in an afternoon. It's gotten even easier than that since then.
And hell, you should be using Chef to set up your LDAP master and slave. So once you have the config correct once, you can bring up another machine without trouble when needed.