How can one make up for missed fasts from over ten years ago, with regret and not knowing their exact number?

You must repent to Allah for delaying the making up of the fasts, then calculate all the days you missed. If you cannot be certain, then make an effort to determine the number and make up what will absolve you of responsibility. You must also feed a poor person for each day the making up of the fasts was delayed, unless you were ignorant of the impermissibility of the delay.