In answering the question how much weight can I lose, I think we should really change the question slightly and say “How much weight should I lose?” after all that obviously depends on where you were starting out from.
If you're very fat then obviously you can lose a lot of weight, that's only normal. On the other hand, if you're fairly thin and you're only slightly overweight, well it's an obvious answer, you can't lose as much.
But really the final answer to that question is within reason you can lose as much weight as you want to. You are the limiting factor, nobody else. You have to decide exactly what type of body shape you want and then make sure that you do everything in your power to get it. Now that obviously means that you need to diet but it also may mean you need to add exercise into the mix as well.
The fact is as I said earlier, you are the limiting factor, you are in charge nobody else. Now of course you can delegate some of that power if you like to your doctor but really, it's you who should take the responsibility. You have to remember that you are only one of his patients but you are all you have. You're your own person and there's nobody else in charge of you nor there be anybody else in charge of you.
The bottom line is it's up to you to make the ultimate decision. It's up to you to carry it through as well. So how much weight can I lose? As much as you want to just make sure you're in charge.