Input Checkbox List - Limit Number of Items Selected JS

I have a multiple select Input Checkbox List and I want to create a JS function for an onClick button that when the user selects more than 10 items in the checkbox list, it will reject the action but if they have fewer than 10 it will do another function which I have already coded.

I want to know how to get the number of elements selected using Javascript, I'm having a really tough time doing this.