How to count array of HTML element in laravel

I have arrays of radio buttons in my view

<input type="radio" id="s20" name="radio[1]" value="5" { { old('radio.1') ? checked="checked" : '' } } />
...

I need to get their count to pass it to a my PostRequestValidator via <input type="hidden" value="count" name="count" />

via Chebli Mohamed