Every time you checkout, you get a new order, and a new order number .Your recent orders when logged in will show just the last one, and you click the 'see all orders' button there to see the complete list of what you've ordered.
All lines in a cart are what you are checking out, so add all the items from the same range you want, then click the checkout button and confirm, and it will create an order for those items.
Then your cart will be empty, and add the next lot, etc - hope that 's clear!?