Ob/gyn prescribed L-arginine suppliment for my wife, who has 12kg increase since conception. Is it really required having this much a weight gain? "

Averages. A woman of average weight before pregnancy should gain 25 to 35 pounds during pregnancy. Underweight women should gain 28-40 pounds during pregnancy. Overweight women may need to gain only 15-25 pounds during pregnancy. You may need to gain more or less weight, depending on what your health care provider recommends.
No worries. I don't think i answered your question the first time. L-arginine is safe to use in pregnancy. In your situation, it's not absolutely required but is more of an adjunct. If you are uncomfortable using it, you don't have to use it. Have a conversation with your OB though. Good luck.