example using count in TF

gistfile1.txt

# define the network interfaces in a count block for the vm's you want to assign them to

resource "azurerm_network_interface" "network-interface" {
  name = "interface-number-${count.index}"
  count = 3
  # ...
}

# The above resource will generate three network interfaces

# Now create a resource block for 3 vm's and when we get to defining the network interface id's
# we use element to select the interface id from the above resource block

resource "azurerm_virtual_machine" "foo" {
  # ...
  count = 3
  network_interface_ids = ["${element(azurerm_network_interface.network-interface.*.id, count.index)}"]
  # ...
}