Depends on how likely you are to have a seizure generally.
Say you have a probability P to have a seizure.
Wellbutrin will increase that probability by W.
Meth will increase that probability by M.
So your probability of a seizure whilst on both meth and wellbutrin will be your base probability multiplied by however much bupropion and meth would increase the risk in your case (P*W*M)--this varies on an individual basis. So nobody can necessarily determine whether or not a seizure on that combination would be likely for you, but there will be an increased risk for one.