There is no allowance. You can eat as many or as few as you want. It is all very personal and you need to set your own limits. If you have a meter, use it to test before eating and a couple of hours later. That will tell you if there were too many carbs in the meal. The idea is to keep the rise from before to after as low as you can. Any more than 2mmol/l and there were too many carbs. In fact, it is preferable to keep any rise even lower than that. The way to go is to "eat to your meter" and keep a food diary, record your levels alongside, and try to learn from that.