It looks like a quirk of your theme (“X Theme”?). In x/framework/css/site/woocommerce/ethos.css it sets the z-index of those quantity selectors to -1, which seems a little unusual to me.
To work around it, can you try creating a tribe-events.css stylesheet (full instructions can be found in our Themer’s Guide, but basically would live in your theme something like this: themes/your-theme/tribe-events/tribe-events.css) and add to following code:
.tribe-events-tickets .woocommerce .quantity input[type="number"] {
z-index: 10;
}
Does that help?