Hello you can create an on-submit client script to check if there is more than 1 attachment when the request is submitted or not.

Here is a sample:

function onSubmit() {
    var answer =true;
  if (g_form.getValue('your_field') == 'no')
{
    
    if(this.document.getElementsByClassName('file-list-wrap').length == 0) {
 g_form.addErrorMessage('PLEASE ATTACH THE REQUIRED FILE ');
        answer= false;
    } 
    }
return answer;
}