You can add a virtual network interface to a virtual machine using a subclass of VirtualEthernetCard, you can set the addressType to Manual, Generated, or Assigned. If you choose Assigned, you can specify a MAC address explicitly.
For more information about virtual devices in general, see Adding Devices to Virtual Machines.
The number of virtual network interfaces depends on the hardware version you specify for a virtual machine. Hardware version 7 virtual machines support up to ten virtual NICs. Hardware version 4 virtual machines support up to four virtual NICs.