Take a look at this post, for sending 2 values to ThingSpeak.
The WebHook Definition handles "up-to" all 8 ThingSpeak Fields from each publish.
The common use case (for myself anyway) is to send the ThingSpeak API Write key with every publish.
That way, you will only need 1 webhook to send data to any of your ThingSpeak Channels.
Or you can save the API Write Key inside each Webhook Definition (as it appears you are doing now) and not publish the API Write Key each time, but you will need a separate Webhook for every channel with that method.