You can enable up to a maximum of 32 1 Gbps and 16 10 Gbps Ethernet ports on an ESXi host. The maximums are governed by the make/model/driver/feature of the NIC cards and their combinations. For instance, you have up to 32 Broadcom 1 GB Ethernet ports using a tg3 driver and NetQueue disabled, but the same NIC with NetQueue enabled can only present 16 ports. If you were to use a combination of 10 GB and 1 GB Ethernet ports, then only 16 10 GB and four 1 GB ports could be enabled.

Refer to the Networking maximums section on page 14 of the Configuration Maximums Guide for vSphere for more details:  https://www.vmware.com/pdf/vsphere6/r60/vsphere-60-configuration-maximums.pdf .

Now that ESXi is capable of managing a large number of physical NICs, there should be a method to logically present these NICs to apply configuration policies on them. This is achieved by enumerating the physical NIC with a vmnicX pattern (vmnic0... vmnic32). Also, there is a logic behind the enumeration. The NICs are sequentially enumerated during the ESXi boot process by scanning the PCI bus, slot, and port number. The PCI-id to vmnic  mapping can be found in the /etc/vmware/esx.conf configuration of an ESXi host: