The backplane function here is performed by the physical uplink switches, that is, the organization's network infrastructure. The number of line cards supported then is not determined by the physical capacity of the chassis, but by the VSM. Currently, up to 64 VEMs are supported by a single VSM or an HA pair.

In N1KV VSM, there are two module slots. Either module can act as active or standby. As an effect, the first ESXi host will automatically be assigned to module 1. The ports to which the virtual NIC interfaces connect are virtual ports on Cisco N1KV, where they are assigned a global number. Have a look at the following diagram for more details:

With the challenges presented by virtualization to the demarcation between host and network, N1KV allows the necessary collaboration between NAs and SAs.

Internal fabric helps the supervisors and line cards to communicate in a physical switch. On a similar note, N1KV uses external fabric that is provided by the upstream switches in the physical infrastructure.

The physical switch's backplane helps line cards to forward traffic to each other. However, N1KV does not have a backplane, thus a VEM cannot directly forward packets to another VEM. So it has to forward the packet via some uplink to the external fabric, which then switches it to the destination.

Each VEM in Cisco N1KV is designed in order to prevent loops in the network topology, as using the Spanning-Tree Protocol (STP) will deactivate all but one uplink to an upstream switch, hence preventing full utilization of the uplink bandwidth.

You can bundle the uplinks in a host in a port channel for load balancing and HA.