Use 2 physical ports if you already have the cable, NIC and switch ports available.
Switches have internal caches that have a fixed allocation per port. Use two ports = twice as much cache available. Some NICs have internal caches (per port if multi-port NIC).
Some broadcast traffic can be filtered by the switch, more ports = potentially less broadcast per port.
As traffic reaches maximum throughput, collision re-transmits will increase load, two ports = less traffic per port = less potential collisions.
In most configurations you should be able to run 16 or more IP's on a single NIC without any measurable difference. If you never fill the switch cache, use "noisy" broadcast or multicast protocols, or run the connections over 75% max rated throughput you won't see a measurably difference to justify the cost of NIC, Switch port, and extra cabling.
The best reasons to use multiple ports are:
- security/network segmentation using VLANS, network addressing, routing and/or source/dest/protocol access control on a managed smart switch.
- Debugging/protocol capture and analysis, reduces clutter, less filtering needed.
We usually have 8 to 12 lightly loaded virtual clients running per development virtualization hosting server with no network issues. (Single NIC, Bridged virtuals, 8-12 IP addresses per NIC).