100% of the time, regardless of system size.
I always use dual if not quad nics and I do that for a few reasons. Dual nic's are great for integrators providing a turn key solution with a network switch, and not using a customers network, it allows you to physically seperate the networks at the nic. Cameras on one nic, viewers on another. Option 2, NIC Teaming, that is easily done through windows server 2012, or if your using broadcoms they have a software download that will do it, you just team them up and you will get thoretically 2Gbps. Also it will offer redundancy, so if you lose a nic, or a switch you dont lose your connection. also if you want to get really creative you could set up 4 nics, and designate 2 of them for download, and 2 for upload, but thats just overkill for 99% of things.
There are a few methods of NIC teaming, Switch dependant and Switch Independant. One requires configurations on the switch, one requires the configurations on the NIC.
Below is a basic explanation from Microsoft.
NIC Teaming Overview | Microsoft Docs
Regardless, always use 2, because if you don't need it today you will tomorrow. Nics are cheap, and they will save you headach and give you more options.
Also most of servers available the market (i.e. Dell) will provide Dual NIC as the basic setup
IPVMU Certified | 11/06/14 10:02pm
You don't need a dual port network interface cards to team them. You can team single port cards together if the OS or NIC driver supports it. If your question was why team NIC's, which many people here seem to assume, the reasons are simply to increase bandwith and/or for fault tolernce.
If the question is why use dual port cards at all, it could be a simple matter of taking up less PCIe slots: a dual port card only takes up one PCIe slot.
IPVMU Certified | 11/06/14 11:03pm
When using SAN or NAS storage I prefer to use dual NICs. One for network connection (primarily from the cameras, a little upstream traffic for viewing). The second NIC is for a direct cross-over connection to the outside storage box.
IPVMU Certified | 11/07/14 06:08pm
You could route your cameras through different switches for the sake of redundancy.
or Have segregated network for groups of cameras with specific policy and qos.