How to get string value from an array of string defined in json template using terraform

As you've identified, the destination_cidr_block only accepts a single CIDR block (a string), not multiple CIDR blocks. You need to create a separate aws_ec2_transit_gateway_route for each CIDR block for each route table. You can do this by flattening the map so there's one element for each RT/CIDR combination.

locals {
  route_tables = jsondecode(file("templates/my.json"))
  rt_cidr_blocks = merge([
    for k, rt in local.route_tables:
    { 
      for i, ip_network in rt.ip_network:
      "${k}-${i}" => {
        tgw_rt_id = rt.tgw_rt_id
        ip_network = ip_network
      }
    }
  ]...)
}

resource "aws_ec2_transit_gateway_route" "ip_route" {
  for_each                       = local.rt_cidr_blocks
  destination_cidr_block         = each.value.ip_network
  transit_gateway_attachment_id  = each.key
  transit_gateway_route_table_id = each.value.tgw_rt_id
}

If you want to see what the flattened map looks like now:

output "rt_cidr_blocks" {
  value = local.rt_cidr_blocks
}

Output:

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

blocks = {
  "RT-1-0" = {
    "ip_network" = "1.1.1.0/24"
    "tgw_rt_id" = "tgw-rtb-00128"
  }
  "RT-1-1" = {
    "ip_network" = "1.1.2.0/24"
    "tgw_rt_id" = "tgw-rtb-00128"
  }
  "RT-1-2" = {
    "ip_network" = "1.1.3.0/24"
    "tgw_rt_id" = "tgw-rtb-00128"
  }
  "RT-2-0" = {
    "ip_network" = "1.1.1.0/24"
    "tgw_rt_id" = "tgw-rtb-01f1b"
  }
  "RT-2-1" = {
    "ip_network" = "1.1.2.0/24"
    "tgw_rt_id" = "tgw-rtb-01f1b"
  }
  "RT-2-2" = {
    "ip_network" = "1.1.3.0/24"
    "tgw_rt_id" = "tgw-rtb-01f1b"
  }
}