I aim for fewer than 100 carbs a day, and no more than 30 in a single meal. I usually end up around 50, mostly from veggies.
I came to these figures by experimentation, a la
Blood Sugar 101. More than 30 grams carb, even "good" carbs like whole grains and lentils/beans, will spike my BG after I eat it. I can usually have a single slice of whole grain bread, 1/4 cup of beans/lentils, or an apple. But I can't have a two-bread-slice sandwich AND an apple... that's too much carb in one sitting for my BG.
I highly recommend that you test, test, test, and "eat to your meter". You'll quickly find out which carbs you can tolerate, and how much. (For instance, oatmeal spikes me no matter what kind it is, or how much... so I don't eat it anymore. Beans don't bother me as much, so I include small servings on occasion.)
The 100 grams/day I chose because it's about where I'd be if I ate three meals with 30 grams each (though I don't to that very often). It gives me some wiggle room, but still keeps my BG stable. It's also where I end up losing weight pretty steadily. It's when my carbs begin creeping higher than 100 grams/day that my weight loss stalls or reverses.
So there's what I do, and why. I hope you find what works best for you!