A better question is how much do you have to loose if you are sued.
If you own a house or fancy car, form a LLC today. I would also sign agreements with any partners to use binding arbitration in your home state if there is a dispute. That way neither side can exploit an advantage with expensive lawyers in the event of a dispute.
If you rent an apartment and only have $18 in your checking account, it is less of a risk.
You will certainly want a company before you start having customers.