Why?

Instead of writing same resource block multiple times, create many with one block.

count

Creates same resource N times

locals {
  project = "project-01"
}

resource "aws_subnet" "main" {
  vpc_id     = aws_vpc.my-vpc.id
  count      = 2                            # create 2 subnets
  cidr_block = "10.0.${count.index}.0/24"  # index changes each time

  tags = {
    Name = "${local.project}-subnet-${count.index}"
  }
}

count.index starts at 0:

index 0 → cidr: 10.0.0.0/24  name: project-01-subnet-0
index 1 → cidr: 10.0.1.0/24  name: project-01-subnet-1

Access by index:

aws_subnet.main[0].id   # first subnet
aws_subnet.main[1].id   # second subnet
aws_subnet.main[*].id   # all subnet IDs as list