What is incumbent upon one who experiences multiple wet dreams in Ramadan and does not know their number, and doubts whether he became in a state of major ritual impurity (janāba) on one of those days or not, then experiences a wet dream on the same day? And what should one do who did not pray due to "excessive harm"?

Wet dreams do not invalidate the fast according to the consensus of scholars. Ghusl (definition) (ritual purification) becomes obligatory after them if semen is discharged and you are certain of its discharge. It is not obligatory if there is doubt. If you missed prayers when they were obligatory, you must repent and make them up.