How to make up a Ramadan fast broken due to masturbation

during ramadan i have masturbated in the middle of the day i really regret doing that and i know it's a big sin but i want to know should i fast 120 days or just one day and what else should i do ? i hope you can help me

It is sufficient to make up just one day of fasting if the fast is broken due to masturbation. Nothing else is necessary.

Fatwa from the Qatari Fatwa Authority (Arabic PDF)