<form id="form">
  <input type="number" id="count">
  <button>Сгенерировать</button>
</form>

<div class="controls"></div>
document.getElementById('form').addEventListener('submit', function(event){
  event.preventDefault();
  
  let count    = document.getElementById('count').value,
      controls = document.querySelector('.controls');
  
  controls.innerHTML = '';
  
  for (let i = 0; i < count; i++) { 
    let input = document.createElement("input");
    
    input.setAttribute('type', 'text');
    input.setAttribute('placeholder', 'id = _' + i);
    input.setAttribute('id', '_' + i);
    
    controls.appendChild(input)
  }
})

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External JavaScript

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