There are no numbers or stats on this. Its probably pretty low, but its certainly possible. No one has all the genes mapped out for their entire family so they can be lurking without expressing themselves until you get lucky
Again though if we had something better than finasteride like dutasteride maybe that was safer to take for prevention then male pattern baldness might largely be preventable, and not be a big deal. dutasteride could probably prevent male pattern baldness in most cases for a long long time, but its so risky most people wont touch it until they are already far along in balding. Who wants to take a drug like dutasteride the costs/risks without knowing they even have male pattern baldness?