Depending on the shape of the area to be covered it is common practice to order 7 to 10% extra for off cuts.
I strongly suggest everyone to have few extra boxes siting in the storage – in case you ever need them to replace damaged plank or the area.
I get calls from people almost every day that bought they floor few years ago at different sources and they need to have only small area replaced but they can not find the same product.
All manufacturers come up with new styles and designs every year and what was popular and available in every store few years ago might be something you can not find any more.
So, lots of people end up replacing the floor in the whole house or entire room just because they do not have few planks in their storage.