It is generally very individual.
Some women gain more weight during pregnancy, some gain less. If your body mass index (BMI) is lower or higher than what is defined as normal, work with your midwife and doctor to have your weight gain fit as closely to the parameters recommended as possible - less or more can cause health problems for you and your baby during pregnancy. If you don’t know what your BMI was before pregnancy, find an online tool to help you calculate it (you will need to know your pre-pregnancy weight and your height).
Some general guidelines for weight gain are:
• Underweight: BMI less than 18.5 - weight gain should be 13kg to 18kg
• Normal weight: BMI 18.5 to 24.9 - weight gain should be 11.5kg to 16kg
• Overweight: BMI 25 to 29.9 - weight gain should be 7kg to 11.5kg
• Obese: BMI 30 or more - weight gain should be 5kg to 9kg
These are just informational references and you should talk to your midwife or doctor about your own specific situation.
Eating for two is a myth - during pregnancy you can eat the same diet you had before pregnancy (no more, no less) and in the third trimester add an extra snack. Quality is much more important than quantity.