Check with your database (Oracle) sales rep regarding your licensing concerns. Some database vendors have specific clauses about 'connection multiplexing' - where middleware, e.g. an App Server, is used to provide an application to a greater number of users than the actual connections to the database.
As for your technical question - the database connection pool can be configured for its behavior when all the members of the pool are in use - e.g. block, block with timeout, instant timeout etc. These are all defined in the XML configuration file for your datasource.