I was able to find an answer to this which was more simple than I thought:

{{cars.length}} will show the sum of the number of objects in the array.

I have the following simple handlebarsjs email HTML template:

<script id="emailTemplate" type="text/x-handlebars-template">
    <div>
       Hi {{name }} , you have submitted {{ SUM_OF_ARRAY_OBJECTS}} cars. Thanks.
    </div>
</script>

With the json content:

{
  "name": "john",
  "cars": [
    {
      "registration": "stubReg1",
      "year": "1995"
    },
    {
      "registration": "stubReg2",
      "year": "2000"
    },
    {
      "registration": "stubReg3",
      "year": "2010"
    },
    {
      "registration": "stubReg4",
      "year": "2016"
    }
  ]
}

How can I display the total number of array objects in the SUM_OF_ARRAY_OBJECTS placeholder using handlebars? i.e. 4 in this example.