seta sv_maxclients 12  <-- total number of players incl private clients.
seta sv_privateclients 3  <-- So your total public slots is 9
seta sv_privatepassword "shogo"  <-- the password

To access your server with a password you can either:

1) Put this statement in your autoexec.cfg file:  set password "shogo" 

2) When you access the server and you get a server is full or something message, bring down the console and enter: 
        /password shogo      (or whatever the password is) 

then you'll be granted access. 

Note that in Gamepsy the server will appear to be password protected even if there are public slots. In this case you need to simply attempt to access the server, when promoted for a password, either enter your password or just click ok (if its in the autoexec.cfg file) leaving the fields empty.