Is repentance, along with consistent performance of the Fajr prayer, sufficient for one who has missed it many times, the number of which he does not know, or is it obligatory for him to make them up, and how would one make them up in this case?

If the Fajr prayer is missed unintentionally due to sleep or forgetfulness, it must be made up (qada'). However, if it is intentionally abandoned, it is a grave sin, and some scholars even consider its abandoner to be a disbeliever. One must repent to Allah Almighty for intentionally abandoning prayer. The majority of scholars hold that making up the prayer is obligatory. As for how to make up for someone who does not know the number of missed prayers, they must investigate and make up what brings them certainty.