I highly do not reccomend using 6 instances on one server. Why not just use 6 users (schemas) instead and one instance. The extra overhead of the instance is not required and your databases will be slow as hell as each instance will compete with each other.

Instead of 6 instances just create 6 different users (each one can have it's own tablespace!)

For less management for you as well and easier backups ect..

It is most common and reccomended to have 1 instance per server.
How much resources each instance uses dedpendss on the INITX.ora parameters.