要构造一个jQuery元素，你需要以下结构：

$('<span>', { class: 'numbers', text: figureCount });

或者：

$('<span>').addClass('numbers').text(figureCount);

工作示例

const $isVertical = $('.is-vertical'); // <div class="is-vertical">
const figureCount = $isVertical.find('figure').length; // 3

$isVertical.append($('<span>', { class: 'numbers', text: figureCount }));

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="is-vertical">
  <figure><img src="src.jpg">
    <figcaption class="wp-element-caption">Towers Day<br>NYC</figcaption>
  </figure>
  <figure><img src="src.jpg">
    <figcaption class="wp-element-caption">Towers Day<br>NYC</figcaption>
  </figure>
  <figure><img src="src.jpg">
    <figcaption class="wp-element-caption">Towers Day<br>NYC</figcaption>
  </figure>
</div>

它最终被这个脚本解决了：

var $isVertical = $('.is-vertical'); // <div class="is-vertical">
var $numbersSpan = $('<span>', { class: 'numbers', text: 'Figure 1 of ' + $isVertical.find('figure').length });
$isVertical.prepend($numbersSpan);

// Function to update the current figure number
function updateCurrentFigureNumber() {
  var windowHeight = $(window).height();
  var scrollTop = $isVertical.scrollTop();
  var figureHeight = $isVertical.find('figure').height();
  
  // Calculate the current figure number in view
  var currentFigureNumber = Math.floor((scrollTop + windowHeight * 0.75) / figureHeight) + 1;
  
  // Update the text inside the 'numbers' span
  $numbersSpan.text( + currentFigureNumber + ' / ' + $isVertical.find('figure').length);
}

// Add an event listener to update the current figure number when scrolling
$isVertical.scroll(updateCurrentFigureNumber);

// Call the updateCurrentFigureNumber function initially to set the correct figure number
updateCurrentFigureNumber();