Well, the safe way would be to mirror the current form spec and treat everything as arrays of strings by default.

    <form enctype='application/json'>
      <input type='text' name='fruit' value='apple'>
      <input type='number' name='bottle-on-wall' value='1'>
      <input type='number' name='bottle-on-wall' value='2'>
      <input type='number' name='bottle-on-wall' value='3'>
    </form>


    {
      "fruit": ["apple"],
      "bottle-on-wall": ["1", "2", "3"]
    }

Then add attributes to explicitly give the structure. Since this is a browser spec we can add attributes. Here's the same form but with some attributes giving the explicit encoding.

    <form enctype='application/json'>
      <input type='text' name='fruit' value='apple' enc='string'>
      <input type='number' name='bottle-on-wall' value='1' enc='array/number'>
      <input type='number' name='bottle-on-wall' value='2' enc='array/number'>
      <input type='number' name='bottle-on-wall' value='3' enc='array/number'>
    </form>

    {
      "fruit": "apple",
      "bottle-on-wall": [1, 2, 3]
    }

Dynamically varying JSON structures by form names and number of form elements is definitely going to be a source of bugs and security problems.