The short answer to your question is that you can definitely put in the quantity per item in this template and have it calculate the total for each client—you need to do it in the Order Line Items Table, in the quantity field.
A slightly longer answer: the product catalog and orders template is structured in such a way that there are separate tables for Clients, Client Orders, Order Line Items, and Furniture.
Clients: information on each client (name, shipping address, etc.—linked to Client Orders table)
Client Orders: the orders submitted by a client (fulfill by date, invoice, total order cost, etc.—linked to Client who ordered and the multiple Order Line Items comprising the order)
Order Line Items: the quantity of each Furniture item within a given Client Order (linked to the respective Client Order and Furniture tables)
Furniture: the items/products themselves (linked to the Order Line Items of which they are a part)
At this point, how the template works might make sense to you, but you still might not understand why the tables are set up this way. Basically, the Order Line Items table serves as a junction table to store metadata about the relationship between the Client Order and the Furniture items.
Imagine that there’s no Order Line Items table. If you attempt to directly link the relevant Furniture items to each Client Order, you cannot express anything about the relationship between Client Orders and Furniture, only that a relationship between those two concepts exists. In certain cases, just knowing that there’s a relationship is enough, but in other cases, you need to say something more specific about the relationship itself—like, for example, the quantity of a given item in an order.
For a more detailed explanation of this, you can read this article on representing many-to-many relationships in Airtable, and more specifically, this section of the article diagramming and discussing the product catalog example.