There is no simple way to determine this. It is generally best to buy as much as you can reasonably afford. You should take into account what assets you have that need to be protected. While registering an aircraft to a holding company may provide some benefit, if the claim is bad enough and the attorneys are diligent enough, it may come back to you individually.
Consider also what your passenger exposure is. How many people will be flying with you on average? What is the worst case scenario?