Many ways:
Javascript: document.querySelectorAll('#maindiv button')
JQuery: $('#maindiv button')
var buttons = document.querySelectorAll('#maindiv button')
console.log(buttons.length)
<div id='maindiv'> <button></button> <button></button> <button></button> <button></button> <button></button> </div>
2
solved Count the number of buttons present inside a div