weight depends on various factors.
however it also would depend on your genetics.
you can put on weight jus enough within your BMI.
you need to put on weight in a healthy and slow way.
eat foods rich in omega 3 fatty acids.
eat protien rich food.
also take in food rich in essential minerals like selenium
zinc etc as these are required for normal body maintenance.
also avoid deep fried foods as though they increase weight it increases the bad cholesterol
eat healthy drink adequate water and regular exercise.
do not take too much vitamins
as it has adverse effects.
i hope i have answered your query.