As in all things Oracle the answer is "it depends". Are we talking production, development, testing, training, stand-by, disaster recovery, or what? Is the database 24x7x365 or is it required only during normal business hours and if it goes down can the business wait until the DBA gets in? Is the DBA expected to double as the SA? Is it on UNIX, Windows, Linux, or whatever? How automated is the backup and recovery? Is it OLTP or decision support? Will you be using a COTS product or a home grown application for data entry and retrieval?
I know that that's not too much help but you need to answer some of those question before you can start on "how many DBAs do I need?"