You can have up to 8 v network interface cards (vNIC)
To add a new vNIC open Hyper-V Manager, select a VM, right-click this VM and select Settings
Click at the top Add Hardware, select Network Adapter at the right panel and then click Add.
The new vNIC will be added, like the picture below. Now edit Virtual Switch and any other option and click OK. That was all.
To remove a vNIC, just select the vNIC and you will find a Remove button, a little bit before the bottom. See the picture above.
To add and remove a vNIC with PowerShell you use the add-vmnetworkadapter and remove-vmnetworkadapter:
add-vmnetworkadapter -vmname vm2 -switchname "Realtek RTL"
Use get-vmswitch to find the vSwitch
remove-vmnetworkadapter -vmname vm2 -vmnetworkadaptername "Network Adapter"
3 vNICs with the same name, all 3 have been removed. To list all the vNICs for a VM use the following:
get-networkAdapter -vmname -vm2