How can I count the number of children?

You can use Element.childElementCount

ref: https://developer.mozilla.org/en-US/docs/Web/API/Element/childElementCount

let count = ThatParentElement.childElementCount;

What if you are using this to determine the current selector to find its children so this holds: <ol> then there is <li>s under how to write a selector var count = $(this+"> li").length; wont work..

You can do this using jQuery:

This method gets a list of its children then counts the length of that list, as simple as that.

$("ul").find("*").length;

The find() method traverses DOM downwards along descendants, all the way down to the last descendant.

Note: children() method traverses a single level down the DOM tree.