You can overwrite some things in ReceiptScreenWidget.

1. The get_receipt_render_env function of the ReceiptScreenWidget. This function is used here:

render_receipt: function() {
    this.$('.pos-receipt-container').html(QWeb.render('PosTicket', this.get_receipt_render_env()));
},

var CustomReceiptScreenWidget = ReceiptScreenWidget.include({
    rand_number_for_ticket: function(){
        return 7;
    },
    get_receipt_render_env: function() {
        var order = this.pos.get_order();
        var number = 4;//use js to find rand number
        return {
            widget: this,
            pos: this.pos,
            order: order,
            receipt: order.export_for_printing(),
            orderlines: order.get_orderlines(),
            paymentlines: order.get_paymentlines(),
            random_number: this.rand_number_for_ticket(),
        };
    },
});

2. Overwrite the xml template.

<t t-extend="ReceiptScreenWidget">

<!--place with jquery the rand number-->

<t t-jquery="some_selector" t-operation="after">//or another operation

<span t-esc="random_number"/>

</t>

</t>