:wasalam:
Please see this thread:
https://www.islamicboard.com/prayer/...cs-prayer.html
There are different actions in prayer, some of them are Fardh, some are Wajib and some are Sunnah.
If you miss a Fardh action of a prayer, then the prayer is invalid and you will have to repeat the whole prayer.
If you miss a wajib action, then you have to perform 2 extra Sajdah at the end, known as Sajdah Sahw.
If you miss a Sunnah action then the prayer is valid and you won't have to do anything.
The number of Raka'at to be prayed in each prayer is Fardh. Reciting Surah after Surah Fatiha is Wajib.
Missing, adding or forgetting a rakah are different scenarios and they will have to be explained separately.