This script can be used to delete all your iOS simulators and recreate the ones you want.

clean_up_and_create_sims.rb

require 'pp'
require 'fastlane'
require 'snapshot/reset_simulators'

# Run the script where you have fastlane installed with `bundle exec ruby clean_up_and_create_sims.rb`

def delete_all_iOS_sims
  FastlaneCore::Simulator.delete_all
end

def all_iOS_sims
  FastlaneCore::Simulator.all
end

def all_iOS_devices
  Snapshot::ResetSimulators.devices
end

def all_iOS_runtimes
  Snapshot::ResetSimulators.runtimes
end

def all_iOS_device_types
   `xcrun simctl list devicetypes`
     .scan(/(.*)\s\((.*)\)/)
     .reject { |device|
       device.join.include? "TV" or device.join.include? "Watch"
     }
end

def all_iOS_device_type_names
  all_iOS_device_types.map { |device| device[0] }
end

def filter_runtimes(all_runtimes, os = 'iOS', versions = [])
  all_runtimes.select { |v, id| v[/^#{os}/] }.select { |v, id| v[/#{versions.join("|")}$/] }
end

def all_iOS_versions
 filter_runtimes(all_iOS_runtimes, 'iOS')
end

def create(device_type, os_versions, os_name = 'iOS')
  os_versions.each do |os_version|
    puts "Creating #{device_type[0]} for #{os_name} version #{os_version[0]}"
    command = "xcrun simctl create '#{device_type[0]} #{os_version[0]}' #{device_type[1]} #{os_version[1]}"
    puts command
    `#{command}`
  end
end

# pp all_iOS_versions
# pp all_iOS_device_types
iOS_versions = [
  ["iOS 12.0", "com.apple.CoreSimulator.SimRuntime.iOS-12-0"]
]


iOS_device_types = [
  ["iPhone 4s", "com.apple.CoreSimulator.SimDeviceType.iPhone-4s"],
  ["iPhone 5", "com.apple.CoreSimulator.SimDeviceType.iPhone-5"],
  ["iPhone 5s", "com.apple.CoreSimulator.SimDeviceType.iPhone-5s"],
  ["iPhone 6", "com.apple.CoreSimulator.SimDeviceType.iPhone-6"],
  ["iPhone 6 Plus", "com.apple.CoreSimulator.SimDeviceType.iPhone-6-Plus"],
  ["iPhone 6s", "com.apple.CoreSimulator.SimDeviceType.iPhone-6s"],
  ["iPhone 6s Plus", "com.apple.CoreSimulator.SimDeviceType.iPhone-6s-Plus"],
  ["iPhone 7", "com.apple.CoreSimulator.SimDeviceType.iPhone-7"],
  ["iPhone 7 Plus", "com.apple.CoreSimulator.SimDeviceType.iPhone-7-Plus"],
  ["iPhone 8", "com.apple.CoreSimulator.SimDeviceType.iPhone-8"],
  ["iPhone 8 Plus", "com.apple.CoreSimulator.SimDeviceType.iPhone-8-Plus"],
  ["iPhone SE", "com.apple.CoreSimulator.SimDeviceType.iPhone-SE"],
  ["iPhone X", "com.apple.CoreSimulator.SimDeviceType.iPhone-X"],
  ["iPhone XR", "com.apple.CoreSimulator.SimDeviceType.iPhone-XR"],
  ["iPhone XS", "com.apple.CoreSimulator.SimDeviceType.iPhone-XS"],
  ["iPhone XS Max", "com.apple.CoreSimulator.SimDeviceType.iPhone-XS-Max"],
  ["iPad 2", "com.apple.CoreSimulator.SimDeviceType.iPad-2"],
  ["iPad Retina", "com.apple.CoreSimulator.SimDeviceType.iPad-Retina"],
  ["iPad Air", "com.apple.CoreSimulator.SimDeviceType.iPad-Air"],
  ["iPad Air 2", "com.apple.CoreSimulator.SimDeviceType.iPad-Air-2"],
  ["iPad (5th generation)",
   "com.apple.CoreSimulator.SimDeviceType.iPad--5th-generation-"],
  ["iPad Pro (9.7-inch)",
   "com.apple.CoreSimulator.SimDeviceType.iPad-Pro--9-7-inch-"],
  ["iPad Pro (12.9-inch)", "com.apple.CoreSimulator.SimDeviceType.iPad-Pro"],
  ["iPad Pro (12.9-inch) (2nd generation)",
   "com.apple.CoreSimulator.SimDeviceType.iPad-Pro--12-9-inch---2nd-generation-"],
  ["iPad Pro (10.5-inch)",
   "com.apple.CoreSimulator.SimDeviceType.iPad-Pro--10-5-inch-"],
  ["iPad (6th generation)",
   "com.apple.CoreSimulator.SimDeviceType.iPad--6th-generation-"]
]

oses = [
  # ["iOS 10.3", "com.apple.CoreSimulator.SimRuntime.iOS-10-3"],
  # ["iOS 11.4", "com.apple.CoreSimulator.SimRuntime.iOS-11-4"]
]
devices = [
  ["iPhone SE", "com.apple.CoreSimulator.SimDeviceType.iPhone-SE"],
  ["iPhone 8", "com.apple.CoreSimulator.SimDeviceType.iPhone-8"],
  ["iPhone X", "com.apple.CoreSimulator.SimDeviceType.iPhone-X"],
  ["iPad Air", "com.apple.CoreSimulator.SimDeviceType.iPad-Air"],
]

os_devices_pairs = [ # [os: [device]]
  [
    ["iOS 11.4", "com.apple.CoreSimulator.SimRuntime.iOS-11-4"], # os
    [ # devices
      ["iPhone 8", "com.apple.CoreSimulator.SimDeviceType.iPhone-8"],
      ["iPhone X", "com.apple.CoreSimulator.SimDeviceType.iPhone-X"],
    ]
  ],
  [
    ["iOS 10.3", "com.apple.CoreSimulator.SimRuntime.iOS-10-3"], # os
    [ # devices
      ["iPhone SE", "com.apple.CoreSimulator.SimDeviceType.iPhone-SE"],
      ["iPad Air", "com.apple.CoreSimulator.SimDeviceType.iPad-Air"],
    ]
  ]
]


# Uncomment the next line to delete all iOS sims
# delete_all_iOS_sims

# Edit os_devices_pairs to create the devices you want
# os_devices_pairs.each do |os_devices|
#   os = os_devices[0]
#   devices = os_devices[1]
#   devices.each do |device|
#     create(device, [os], 'iOS')
#   end
# end