You can configure up to 4 virtual network interfaces from the Networking page of the New VM wizard. To configure more than 4, go to the VM's Networking tab after it has been created and add them from there.
By default, an automatically-created random MAC address will be used for all virtual network interfaces. To enter a different MAC address, click Properties and enter a new address in the Virtual Interface Properties dialog box, using hexadecimal characters in the form aa:bb:cc:dd:ee:ff.
You can use the Networking tab to change the VM's virtual networking configuration later, if required; see the Help topics in the Configuring VM networking folder.