terraform-subnet-size-calculator.tf

locals {
  vpc_cidr = "172.31.0.0/16"
  region   = "us-east-1"
  azs      = ["a", "b", "c"]
  subnets = {
    "private" = {
      all = 2,
      az  = 2,
    },
    "public" = {
      all = 4,
      az  = 2,
    },
    "database" = {
      all = 6,
      az  = 2
    },
    "intra" = {
      all = 6
      az  = 2
    },
  }
  cidrs = cidrsubnets(local.vpc_cidr, [for k, v in local.subnets : v.all]...)
  network_size = {
    "255.255.0.0"     = 65534, # /16
    "255.255.192.0"   = 16382, # /18
    "255.255.240.0"   = 4094,  # /20
    "255.255.252.0"   = 1022,  # /22
    "255.255.254.0"   = 510,   # /23
    "255.255.255.0"   = 254,   # /24
    "255.255.255.192" = 62,    # /26
  }
}
output "subnets" {
  value = {
    "${local.vpc_cidr} (${local.network_size[cidrnetmask(local.vpc_cidr)]} ips)" = {
      for n in keys(local.subnets) : n => {
        "${local.cidrs[index(keys(local.subnets), n)]} (${local.network_size[cidrnetmask(local.cidrs[index(keys(local.subnets), n)])]} ips)" = {
          for sidx, s in cidrsubnets(local.cidrs[index(keys(local.subnets), n)], [for aidx, i in toset(local.azs) : local.subnets[n].az]...) : "${local.region}${local.azs[sidx]}" => "${s}  (${local.network_size[cidrnetmask(s)]} ips)"
        }
      }
    }
  }
}

# $ terraform plan 
#
# + subnets = {
#      + 172.31.0.0/16 (65534 ips) = {
#          + database = {
#              + 172.31.0.0/22 (1022 ips) = {
#                  + us-east-1a = "172.31.0.0/24  (254 ips)"
#                  + us-east-1b = "172.31.1.0/24  (254 ips)"
#                  + us-east-1c = "172.31.2.0/24  (254 ips)"
#                }
#            }
#          + intra    = {
#              + 172.31.4.0/22 (1022 ips) = {
#                  + us-east-1a = "172.31.4.0/24  (254 ips)"
#                  + us-east-1b = "172.31.5.0/24  (254 ips)"
#                  + us-east-1c = "172.31.6.0/24  (254 ips)"
#                }
#           }
#          + private  = {
#              + 172.31.64.0/18 (16382 ips) = {
#                  + us-east-1a = "172.31.64.0/20  (4094 ips)"
#                  + us-east-1b = "172.31.80.0/20  (4094 ips)"
#                  + us-east-1c = "172.31.96.0/20  (4094 ips)"
#                }
#            }
#          + public   = {
#              + 172.31.128.0/20 (4094 ips) = {
#                  + us-east-1a = "172.31.128.0/22  (1022 ips)"
#                  + us-east-1b = "172.31.132.0/22  (1022 ips)"
#                  + us-east-1c = "172.31.136.0/22  (1022 ips)"
#                }
#            }
#        }
#    }

Excellent work! Where do you find the expansion documentation for []...?

Found it: https://www.terraform.io/language/expressions/function-calls#expanding-function-arguments