I agree with the previous posters who state that it really depends on your starting weight how much or little you should out on.
But Alan has a very good point, too. Don't worry too much about your weight gain. I started out overweight (BMI 29) and have only put on just over 7 kg so far. You have no idea how many people say: what, you're 34 weeks? But you're tiny! You should eat more! You don't want an underweight baby!
Well, my midwives seem to think that the baby is growing very well and it keeps measuring just on target. And that's all that counts to me.
My philosophy about this is that my body will tell me what it needs. I eat when I'm hungry and try to eat healthy food. But I do indulge every now and then. I don't count calories and I don't do a lot of exercise. Just walking my dog, which I better do now.

Don't worry, no matter what you do, people will either comment on you putting on too much or too little. It seems like you just can't ever get it right.