Is ghusl required after the emission of semen upon watching pornographic movies, and is the fast of making up for last Ramadan valid if one fasted without performing ghusl in this situation, and is ghusl required for each instance of semen emission during the day, or is one ghusl sufficient?

It is forbidden to watch pornographic films, and one must hasten to repent from that sin.

As for your fasting: If the viewing occurred during your fast and semen was discharged, then the fast is invalidated and requires a make-up day. If it was not during the time of fasting, then your fast is valid.

As for prayer: Your prayers without ghusl after the discharge of semen are not valid, and you bear sin if you are not excused by ignorance. You are required to make up the missed prayers according to the majority of scholars, although there is another opinion that waives the make-up for the ignorant. If semen is discharged multiple times, one ghusl is sufficient, as long as no further discharge occurs after the first ghusl.