Well, there are really three answers here:
1. As much as you want. Most traditional diets around the world are based on a starch and filled in with vegetables, fruit, meat, fish, dairy and eggs as they are available and affordable. The Italian diet includes lots of pasta and the Peruvian diet has been based on potatoes. The Japanese eat tons of white rice. As Sophia Loren said, "Everything you see here I owe to spaghetti."
There was a segment on
The Potato Diet on the Today Show recently about The Potato Diet. As the man who did the experiment says, it's not meant to be the next fad diet, but an experiment to show that potatoes are a nutritious food and part of a healthy diet. It's not the first time something like this has been done and the results were the same.
When I'm feeling poor, my diet becomes heavily potato-based filled in with the other things as I can afford them. Never fails -- I lose weight.
2. 1/4 of your plate when pasta or potatoes are a side dish. The remainder of the plate is 1/4 meat/fish/poultry and 1/2 vegetables.
3. 1/3 of your plate when the main dish is a mixed dish. The remaining 2/3 should be vegetables and/or fruit.
But, as KCCC said, one plate of food divided as you choose. It
can be very heavy on potatoes or pasta if you choose.