HTMLTableSectionElement: свойство rows

<table>
  <thead>
    <th>Col 1</th>
    <th>Col 2</th>
    <th>Col 3</th>
  </thead>
  <tbody>
    <tr>
      <td>X</td>
      <td>Y</td>
      <td>Z</td>
    </tr>
  </tbody>
</table>
<button id="add">Add a row</button>
<button id="remove">Remove last row</button>
<div>This table's body has <output>1</output> row(s).</div>

// Obtain relevant interface elements
const bodySection = document.querySelectorAll("tbody")[0];
const rows = bodySection.rows; // The collection is live, therefore always up-to-date
const rowNumberDisplay = document.querySelectorAll("output")[0];

const addButton = document.getElementById("add");
const removeButton = document.getElementById("remove");

function updateRowNumber() {
  rowNumberDisplay.textContent = rows.length;
}

addButton.addEventListener("click", () => {
  // Add a new row at the end of the body
  const newRow = bodySection.insertRow();

  // Add cells inside the new row
  ["A", "B", "C"].forEach(
    (elt) => (newRow.insertCell().textContent = `${elt}${rows.length}`),
  );

  // Update the row counter
  updateRowNumber();
});

removeButton.addEventListener("click", () => {
  // Delete the row from the body
  bodySection.deleteRow(-1);

  // Update the row counter
  updateRowNumber();
});