using count/count.index to spin up multiple instances with EIPs.

gistfile1.py

######################################################################
## Set up NAT instances
###
resource "aws_instance" "nat" {
    ami                         = "${var.aws_nat_ami}"
    count                       = 2
    associate_public_ip_address = true
    availability_zone           = "${lookup(var.az, count.index)}"
    instance_type               = "t2.micro"
    key_name                    = "${var.aws_key_name}"
    security_groups             = ["${aws_security_group.sg.id}"]
    source_dest_check           = false
    subnet_id                   = "${aws_subnet.pub1.id}"
    tags                        = {
                                    "Name"    = "${var.env}-nat-${count.index}"
                                    "owner"   = "${var.owner}"
                                    "email"   = "${var.email}"
                                    "group"   = "${var.group}"
                                    "env"     = "${var.env}"
                                  }
    connection {
       user     = "ec2-user"
       key_file = "~/.ssh/${var.aws_key_name}.pem"
       agent    = false
    }				  
    provisioner "remote-exec" {
        inline = [
          "sudo iptables -t nat -A POSTROUTING -j MASQUERADE",
          "echo 1 |sudo tee /proc/sys/net/ipv4/conf/all/forwarding > /dev/null",
        ]
      } 
}

output "nat" {
   value = "${aws_instance.nat.${count.index}.public_ip}"
}

resource "aws_eip" "nat-eip" {
   instance   = "${aws_instance.nat.${count.index}.id}"
   vpc        = true
   depends_on = ["aws_instance.nat.$count.index}"]
}

output "nat-eip" {
   value = "${aws_eip.nat-eip.${count.index}.public_ip}"
}

A few things:

1. If var.az is a list, you should use element instead of lookup. If it is a map then it will need numerical keys ranging from 0 to the value of count (in this case, 2). Just a heads-up because I can't see the value of ${var.az} here 😃
2. aws_eip.nat-eip will need a count equal to the aws_instance.nat's count.
3. I don't think you need the depends_on for the EIP, since it should depend on it implicitly through instance.
4. I think interpolations inside of interpolations works, but I personally don't trust it so I'd rewrite the instance line to be instance = ${element(aws_instance.nat.*.id, count.index)}
5. Similarly for your output, you can't have lists, so I'd rewrite it to be a comma-separated list: value = ${join(",", aws_eip.nat-eip.*.public_ip)}