<div id="container">
  <input type="text" name="input1" placeholder="输入框1">
</div>
<button id="addButton">添加输入框</button>

var container = document.getElementById("container");
var addButton = document.getElementById("addButton");

addButton.addEventListener("click", function() {
  var newInput = document.createElement("input");
  newInput.type = "text";
  newInput.name = "input" + (container.children.length + 1);
  newInput.placeholder = "输入框" + (container.children.length + 1);
  
  container.appendChild(newInput);
});