The amount of carbs to aim for varies from person to person. We all have different tolerance levels, different amounts of insulin resistance, different medications, different exercise regimes, and different bodies. A blood glucose meter will show you how many carbs your body can cope with. On this forum it varies from almost no carbs up to about 100g.
It is highly unlikely you will get one from your doctor. They are only normally prescribed for those on insulin or drugs that can cause hypos. (low blood sugar). Metformin is not one of these. The vast, vast, vast majority of us have to self fund, so I suggest you buy yourself a meter, some lancets and loads of testing strips (you will need loads). We can point you in the direction of the meters with the cheapest strips as they vary from about £8 for 50 up to as much as £30 for 50. So be careful.
If you do get a meter, and that is very advisable, we can help you to use it to your best advantage.
By the way, it isn't all about losing weight. It is about controlling your blood sugar levels. Losing weight helps a lot, but what your blood sugar levels are doing is important. On a low carb diet, you can do both.