Customizer: Update Preview instantly when typing into a number input field

This works for me:

jQuery(document).ready(function(){
    setTimeout(function() {
        jQuery('input[type="number"]', window.parent.document).on('keyup', function(){
           window.parent.jQuery(this, window.parent.document).trigger('change'); 
        });
    }, 0);
});

Use customize_preview_init to enqueue this js.