At present, when the '+' is clicked, a second NIC can be added, but there is no option to add a 3rd/4th. The current VM config must be saved, then edited again, and so-on until the desired number is reached.

This is in contrast to vDisks, where any number can be assigned to a VM in one go. (the '+' icon is present on each new VD, as well as the '-')

 

While we're at it, it would be lovely if we could select 'model type' from the UI as well. Manually changing ESX VMs to vmxnet3 can get tedious...

Install the Community App Plugin, then install the version of Virt Manager maintained by djaydev (The other version is no longer maintained)...  This will give you all the things  you think are missing...