In order to get the latest build number from App Store or TestFlight, you will need the App Store credentials as well as the Application Apple ID. This is an automatically generated ID assigned to your app and it can be found under General > App Information > Apple ID under your application in App Store Connect.

1. Add the Application Apple ID to the codemagic.yaml as a variable
2. Add the script to get the latest build number using app-store-connect and configure the new build number using agvtool.
3. Your codemagic.yaml will look like this:

   workflows:
     ios-workflow:
       name: iOS Workflow
       integrations:
         app_store_connect: <App Store Connect API key name>
       environment:
         vars:
           APP_STORE_APPLE_ID: 1555555551
       scripts:
         - name: Increment build number
           script: | 
             #!/bin/sh
             cd $CM_BUILD_DIR
             LATEST_BUILD_NUMBER=$(app-store-connect get-latest-app-store-build-number "$APP_STORE_APPLE_ID")
             agvtool new-version -all $(($LATEST_BUILD_NUMBER + 1))
         - name: Build ipa for distribution
         script: | 
           # build command