<div id="row">
<div class="page-block">
Страница 1
</div>
</div> 
<button class="addBtn">+Страница</button>
<button class="addBtn">Готово</button>
.page-block {
  width: 200px;
  height: 140px;
  padding: 10px;
  border: 2px solid #888;
  color: #444;
  box-sizing: border-box;
  margin-bottom: 0;
}
.addBtn {
  background: #eee;
  color: #444;
  padding: 10px;
  margin: 10px;
  border: 2px solid #888;
  cursor: pointer;
  border-radius: 5px;
}
const addBtn = document.querySelector('.addBtn');
var blockNumber = 2;
addBtn.onclick = () => {
  // Создаем блок и добавляем номер
  const createBlock = document.createElement('div');
  createBlock.className = 'page-block';
  createBlock.innerText = 'Страница ' + blockNumber;
  blockNumber++;
  addBtn.insertAdjacentElement('beforebegin', createBlock);
}

External CSS

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External JavaScript

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