document.querySelector("#mg-161-xf_1").addEventListener('click', function(e) {
    if(e.target.tagName=='LI') {
        e.target.classList.toggle("selected");
        len()
    }
});
                   
function len() {
    var bt = document.querySelector("#g-161-xf_1");
    bt.textContent = bt.textContent.replace(/\d+/, document.querySelectorAll("li.selected").length);
}
                   
len();

.selected { color: red;}; - баг.