How frequently do you have to have panic attacks before it is a problem, for example, becomes a medical condition or prevents you from driving?

That depends: Severity is also an important issue. It is usually best to pull over to the side of the road to calm yourself if you are driving and have a panic attack. If you can't focus well on the task at hand - you shouldn't be driving. If continous panic when driving - would stop driving until symptoms improve. Discuss particulars with your doctor.