However, it's darn frustrating to figure out how to enable it as all of the googling and manuals indicate you just enable it by selecting settings within the VM.
What they fail to mention though (but is covered in the built-in help in VirtualBox), is that you must first create a device for this host-only networking to use. From the main VirtualBox window, choose Preferences (below the File) menu. That will bring up the global preferences for the VirtualBox hypervisor. Click "Network" and then the "+" sign to add a host-only network device (typically vboxnet0).
It's possible that some versions of VirtualBox create one of these at install time, but on Ubuntu, such is not the case. Now you can create a number of virtual machines and put them on the same nic. Your host OS will also now show that same nic. Here's an example from my laptop's OS:
medberry@handsofblue:~$ ip a show dev vboxnet0
link/ether 0a:00:27:00:00:00 brd ff:ff:ff:ff:ff:ff
inet 192.168.56.1/24 brd 192.168.56.255 scope global vboxnet0
inet6 fe80::800:27ff:fe00:0/64 scope link
valid_lft forever preferred_lft forever
and that same vboxnet0 is now an option when you select host-only networking in the vm: