JSFiddle - React, Tailwind, and code Playground
HTML
<input id="result" type="text" />
JavaScript
let current_qty = 8;
let container_size = 18;
let container_max = 6;
let max = 2;
let new_qty = Math.min( current_qty - (container_size - container_max), max);
new_qty = new_qty > 0 ? new_qty : 0;
const elem = document.getElementById("result");
elem.value = new_qty;