Itook my final exam which is worth 12% of my grade, my final had 50 questions, but i think my teacher will grade only 25 questions from my final. i have 73.2% in my class. how much can my final affect my grade? how many points can i get from my final if i passed it?

If you get 50 questions, that means you get all 12%.
You add that you're current percentage: 73.2% + 12% = 85.20%

However, if you think you'll only get 25, then you'll get:
(25/50) (12%) = 6%

You add this to your current percentage:
73.20% + 6% = 79.20%

Refer to you passing score and see if 79.20% gives you a passing mark.