The amount of cover your business needs depends entirely on the type of services it provides, so there is no general rule of thumb for how much cover your business will need. You know your business best and the sort of exposure it faces.
When deciding on the amount of Professional Indemnity cover to have you should consider the value of the work you are doing for your clients and the potential financial impact on them should it go wrong. Other than that, it's advisable to buy as much cover as you can afford.
Sometimes your client will set a minimum limit of the indemnity they require before you can work with them. This can depend on the specific nature of the services you've been contracted to provide, or they could have a set limit for all their service providers. Many UK government departments, for example, will require their contractors and consultants to have at least £5 million of cover.
It is also worth considering taking retroactive cover for work that you've already completed before the policy commences. This means that your Professional Indemnity Insurance policy will cover you for any claims arising out of work you completed in the past.