you can have as many database as you can ( as long as u have resources).
every database needs a unique sid.
in UNIX:
export your ORACLE_SID everytime u want to startup / shutdown or use the dbstart script along with oratab, 
so one script can startup/shutdown all databases
in WINDOWS:
create ur own service for every database ( ORACLE_SID).
use ORADIM to do this.

 rem Delete the old service
         ORADIM -DELETE -SID test -SRVC servicename

 rem Create a new services with the STARTMODE AUTO property
         oradim -new -sid TEST -intpwd ORACLE -startmode AUTO -pfile C:orantadminTESTpfileinitTEST.ora