It depends on your age BUT it also depends on your situation. Typically, it's best to get to a 19 BMI or above after recovering from an eating disorder or starvation, but you should really check things out with your doctor for a better opinion. Good luck! And also, if you need to gain but don't want to be "chubby" (though I doubt you were chubby at that BMI), you can try and gain muscle! Ten pounds of muscle can actually make you look slimmer. Just a thought. But it's good to make sure you have enough fat on your body as well.