I am guessing you're speaking more to a wholesale business where by you are packing and shipping products yourself, rather than a drop-shipping model where 95% of the work is done for you.
It's just about forward planning really, you have to set targets and then know what processes you need to have in place to ensure your business can still run smoothly when reaching those targets. So for example, if you are a one man business, you need to know what your limits are as to how productive you can be.
If your targets are set outside of your own productivity levels, then you need to have planned processes that you can put into place when needed. That could well include bringing on staff to ensure the productivity of the business doesn't suffer.
The last thing you want to happen is to set targets, reach them and then find out that you can't sustain them with your current processes. It's all about good forward planning, knowing where you want to get your business to and what you need in place to ensure you can accomplish those goals.