In SIP trunking the “number of lines” is different to a “phone number or DID”. The number of lines defines the maximum number of simultaneous EXTERNAL calls you can make or receive at any given time. As a general rule, the more customers, employees and daily telephone calls you have, the more phone lines you require to be able to service all incoming/outgoing traffic of your business at peak times.
On the other hand, phone numbers are actual Australian phone numbers (like 08 7089 5566) which your customers can dial to contact you. Multiple DIDs are useful when you want incoming calls to be routed to different persons/departments based on the dialled number. Keep in mind, this has nothing to do with number of lines and how many simultaneous calls you can make/receive in your phone system.