There are several factors that go into choosing a professional indemnity limit. The size of your business, income, number of employees, contractual requirements, legislative requirements and the type of consulting you do.
If you are just starting out, then depending on the nature of your work, you might not need a substantial limit. If you are entering into a contract, the other party may require you to have a specified limit. If you are renewing your practising licence, your government body may require a minimum limit.