Depending on the networking mode that you want you can select one of the options(see Virtual Networking Modes section below) from the drop down box with the label Attached to.

To add a network adapter to your Virtual Machine just select the Enable Network Adapter check box.

If you need to add more network cards then you can do so from command line using VBoxManage modifyvm command. You can configure up to four network cards from Network section of the settings window.

To configure your Virtual Machine network, Open the Virtual machine Settings and select Network Virual Machine Settings → Network Let's look at these networking modes in detail and analyse which networking mode to use and how. You can also configure each network adapter in your Virtual Machine to operate in one of the following modes:

Paravirtualized network adapter (virtio-net).

AMD PCNet FAST III (Am79C973) This is the default.

Every Virtual Machine in your VirtualBox can have up to 8 network adapters and these network adapter can be any of the following six hardware types. Oracle VM VirtualBox gives you several options to configure networking of your virtual machines.

What is virtualbox networking feature