I would go against the MW slightly and say that at first you need to express at least every time your LO feeds, if not as much as possible, just to build your supply up. LO can get the milk out of your breast much better than a pump can do so you will have to do more expressing to get the same amount out as the baby would do....
also some people just find they cannot express but baby feeds quite well (I realise not in your case as you say baby wont latch)
You still need to make sure you're drinking at least 2 litres of fluid a day, and eating plenty of high calorie energy food to ensure your milk is top quality
You dont say how old your lo is, but I'm guessing newborn? If so then its normal to get only 2oz ish out as thats prob all baby is taking per feed as their tummies are tiny tiny tiny!
xxx