Smartsheet isn't designed for inventory management via order forms so you'll likely run into problems trying it via this method.  There's no real way to build in a conditional 'item #1, item #2' without having all of it part of your form.

The response above was referring to building out several columns in your sheet so your form would have capacity for Item #1, item #2, etc.  You'll have issues since every order will show each field by default so they'll see all the extra fields even if they're ordering one item.

I would suggest creating an order form for your customers to use and having them upload the file when they submit an order.  You would then confirm the order and enter the data into your sheet for accuracy.  It's probably the easiest way to use Smartsheet for this purpose without creating extra work for your customers entering in an order.