Edit in JSFiddle

function activar(event){
	event.preventDefault();
	var inputs_form = document.querySelectorAll('.input_servicio');
  for(var i = 0; i < inputs_form.length; i++){
  	if(inputs_form[i].disabled){
    	inputs_form[i].disabled = false;
    }else{
    	inputs_form[i].disabled = true;
    }
  }
}

<form action="#" id="demo">
<button onclick="activar(event)">Activar</button>
  <p><label><sub>Item</sub><br><input type="text" name="servicio[]" disabled="true" class="input_servicio"></label></p>
  <p><label><sub>Item</sub><br><input type="text" name="servicio[]" disabled="true" class="input_servicio"></label></p>
  <p><label><sub>Item</sub><br><input type="text" name="servicio[]" disabled="true" class="input_servicio"></label></p>
  <p><label><sub>Item</sub><br><input type="text" name="servicio[]" disabled="true" class="input_servicio"></label></p>
  <p><label><sub>Item</sub><br><input type="text" name="servicio[]" disabled="true" class="input_servicio"></label></p>
  <p><label><sub>Item</sub><br><input type="text" name="servicio[]" disabled="true" class="input_servicio"></label></p>
</form>