I am in my 18th week now. I had severe sickness and vomiting starting from my 5th week. Initially I used to vomit 10-12 times a day. I am taking Zofran twice a day and half a unisom in the evening.

Nowadays, my vomiting has reduced to 2-3 times a day. But when I not vomiting I am terribly nauseaus. I am still on the meds.

Total I have put on 9 pounds, but I still don't feel like eating anything except for fuits.

Any idea when this vomiting and nausea will go away? Is my weight gain too less? Is it going to affect my baby?

It may go away next week, you may continue to feel some nausea throughout your pregnancy. Most women will find it subsides in their second trimester, so hopefully you will start to notice some change before long.
You weight gain is fine, especially considering your experience with "morning" sickness.

Since it is under control... even if not completely eliminated, baby will be fine. Eat those foods that you can right now, experiment now and then with other foods to see what you may be able to add to your intake, and be sure you have plenty of liquids throughout the day.

Hope you feel better soon!