We have a promo sales event on our store which is if you buy 1 Keyboard, you Get 20% off, if you Buy 2 Keyboards, Get 30% off, Buy 3 Keyboards, Get 40% off. Given the Keyboard's product price is USD 199.99. There is a maximum limit of only 6 keyboards where you can apply discount, if you buy more than that, no more additional discounts will be applied.

Can anyone help me to create an algorithm to apply the discount and to compute the total amount if customer buys 1 up to 6 or more of this specific product?

Buy 1 Keyboard  = 20% discount
Buy 2 Keyboards = 30% discount
Buy 3 Keyboards = 40% discount

This was the first code I tried to solve the our issue but I was getting incorrect results:

let cart = [];
let disc_amount;
let total_amount;

discFor1 = 0.2;
discFor2 = 0.3;
discFor3 = 0.4;
discFor4 = 0.6;
discFor5 = 0.7;
discFor6 = 0.8;

const keyboard = {
    id: "1001",
    name: "Yamaha PSR-E273 Keyboard",
    reg_price: 199.99,
    disc_price: 0
};


const addKeyboardToCart = () => {
    cart.push(keyboard);
};

const getDiscount = (cart) => {
    if (cart.length < 1) {
        console.log("Please add an item");
    }

    if (cart.length === 1) {
        disc_amount = keyboard.reg_price * discFor1;
    } else if (cart.length === 2) {
        disc_amount = keyboard.reg_price * discFor2;
    } else if (cart.length === 3) {
        disc_amount = keyboard.reg_price * discFor3;
    } else if (cart.length === 4) {
        disc_amount = keyboard.reg_price * discFor4;
    } else if (cart.length === 5) {
        disc_amount = keyboard.reg_price * discFor5;
    } else if (cart.length === 6) {
        disc_amount = keyboard.reg_price * discFor6;
    } else if (cart.length > 6) {
        disc_amount = keyboard.reg_price * discFor6;
    }

    return disc_amount;
};

const getTotalAmount = (disc_amount, cart) => {
    let total_reg_price_amount = 0;
    for (let i = 0; i < cart.length; i++) {
        total_reg_price_amount += cart[i].reg_price;
    }
    total_amount = total_reg_price_amount - disc_amount;

    return total_amount;
};

addKeyboardToCart();
addKeyboardToCart();
addKeyboardToCart();
addKeyboardToCart();
addKeyboardToCart();
addKeyboardToCart();

getDiscount(cart);

getTotalAmount(disc_amount, cart);

Edit: I'm also trying to implement how to get the best discount for the customer that's why I'm trying to apply the discount dynamically instead of hard-coding the discount amount for every number of items ordered.

Edit: I revised my question and removed additional questions so that the answers can focus more on a specific problem.