First, you need to turn on the pop-up block on that site in your browser.

<button onclick="openWindow()">
  Open Multiple Window
</button>

// function that open the new Window
function openWindow() {
	window.open("https://yahoo.com","_blank").focus();
  window.open("https://bing.com/","_blank").focus();
}

you can modify yourself. You can use for loop to open multiple window.