This always comes down to what is required from you or what suits the business' needs. For example, if you are looking to attend markets and pop-ups you will more than likely need the $20,000,000 limit. If you use certain council grounds, properties or are involved with the Department of Education, you may also need $20,000,000.
Before you purchase, always ask the various parties you are engaged with to see if they are requiring you to have a set limit. Otherwise it comes down to what you think is best for the business and what the business can afford.