If you work in one of the professions for whom Professional Indemnity insurance is a legal requirement, the amount of cover you need may be mandated by your regulatory body. You should check on their website to find out.
Your clients may also specify within their contract the amount of PI cover they require you to have in place to protect the work you do for them.
If the amount of cover you need is not pre-determined, you can work it out by thinking about the size of projects you work on and your potential exposure to risk. We can help you calculate what this might be. Our recommendation would be to buy as much cover as you can afford.