Will my repentance be accepted after committing the abomination of adultery during the sacred months and contracting a venereal disease, given my fear of the disease returning and that my repentance came after the affliction and not before it? And how do I make up for the missed prayers and days of Ramadan, the number of which I do not know?

Praise be to Allah, Who has accepted your repentance for your great sin. You must continue on the path of righteousness, observing acts of obedience, performing obligations, abstaining from prohibitions, and increasing in supererogatory acts. Allah accepts sincere repentance and erases sin, as He says: "And He it is Who accepts repentance from His servants and pardons sins" and His saying: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'"

You must make up the missed prayers and the days of Ramadan (definition) that you broke your fast, along with sincere repentance. If you do not know their exact number, then investigate and make up what will bring you certainty or a strong presumption that you have fulfilled your obligation.