The best way to order multiple personalised items will depend on if you are wanting to have the same personalised message on each bar in the order, or if you would like to have a different name or message on the front of each bar.
For having the same personalisation on every bar the best option is to choose "personalise a single item". This will allow you to create the one personalised item, and will then show a quantity field which can be changed on the preview screen, or in your order basket.
If you choose "personalise multiple items" you can have a different name or message on the front of each bar, and the same message on the back of them all. This option will not show a quantity field, as it will create an order for one bar per name or message added to the front.