With some exceptions (for injuries and major overtraining), the more you train the better. When you first start, 2x a week is probably pretty good. Your body will still be getting used to the abuse it takes doing BJJ and you'll be able to work yourself into it. Coming from 0 ground knowledge, training 2x a week, you'll see a lot of improvement quickly. After you have some training under your belt in the basics, your progress might slow being that you're only going twice a week, but it's still better than nothing. Bottom line, 2x a week is fine for starting off, but down the road you may want to consider bumping it up, especially if you have goals of competition or moving up in the BJJ ranks.