Create multiple EC2 instances using python script (in loop iteration)
The main issue I see is that you are calling Terraform at the end of every python loop. This is creating new "state" file. That will make it really hard to manage what you are creating. This is how I would do it. 

users = []
count = 0
for customer in data['customers']:
    username = customer['email'].split('@')[0]
    instances = customer['instances']
    count += instances
    users.extend([username] * instances)
    #print(Added {} to build list with {} instances.".format(username, instances))
myfunc(count, users)


then I would change...

def myfunc(count, users):

  tf = Terraform(working_dir='/home/ja/terraform-course/demo-2b', variables={'count':count,'INSTANCE_USERS':users})
  tf.plan(no_color=IsFlagged, refresh=False, capture_output=False)
  approve = {"auto-approve": True}
  print(tf.plan())
  print(tf.apply(**approve))
  return


and finally the Terraform.

resource "aws_key_pair" "mykey" {
  key_name = "mykey"
  public_key = "${file("${var.PATH_TO_PUBLIC_KEY}")}"
}

resource "aws_instance" "win-example" {
  ami = "${lookup(var.WIN_AMIS, var.AWS_REGION)}"
  instance_type = "t2.medium"
  count="${var.count}"

  key_name = "${aws_key_pair.mykey.key_name}"
  user_data = &lt;&lt;EOF
&lt;powershell&gt;
net user ${var.INSTANCE_USERS[count.index]} '${var.INSTANCE_PASSWORD}' /add /y
net localgroup administrators ${var.INSTANCE_USERS[count.index]} /add

winrm quickconfig -q
winrm set winrm/config/winrs '@{MaxMemoryPerShellMB="300"}'
winrm set winrm/config '@{MaxTimeoutms="1800000"}'
winrm set winrm/config/service '@{AllowUnencrypted="true"}'
winrm set winrm/config/service/auth '@{Basic="true"}'

netsh advfirewall firewall add rule name="WinRM 5985" protocol=TCP dir=in localport=5985 action=allow
netsh advfirewall firewall add rule name="WinRM 5986" protocol=TCP dir=in localport=5986 action=allow

net stop winrm
sc.exe config winrm start=auto
net start winrm
&lt;/powershell&gt;
EOF

  provisioner "file" {
    source = "test.txt"
    destination = "C:/test.txt"
  }
  connection {
    type = "winrm"
    timeout = "10m"
    user = "${var.INSTANCE_USERNAME[count.index]}"
    password = "${var.INSTANCE_PASSWORD}"
  }

tags {
Name="${var.INSTANCE_USERS[count.index]}"
}
}


What I did was change your code so we now create a list of the users, that match the length of the count. That way we can call Terraform just once, and use the count.index on our users list to make sure we create the correct amount of instances per user.

You can read more about count.index here. I have always found reading and understanding more complicated code makes me better. Try reading some of the modules from the Terraform Registry. The vpc module gives good examples of using count.

If you still need help drop me a comment.