What should one do if they have missed prayers but cannot remember whether or not they have made them up?

Whoever is certain that he has missed prayers and doubts whether he has made them up must perform them again, because the default is that they have not been performed. If he cannot ascertain their number, he must continue to make them up until he is reasonably certain that his obligation has been fulfilled, because if a person's responsibility is certainly engaged by a right, it is not discharged except by certainty.