They don't set a limit - and they're actually pretty cagey about carbs which is a bit frustrating sometimes! On the Fast800 community (the paid for version) they did finally relent and suggest that your day should be 40% protein, 30% fat and 30% carbs.
In the original recipe book they do give the carb counts, which is helpful, but they don't then show how the recipes break down into the macro percentages, so it's difficult to work out. The only way I could work out to do that was to enter everything that you're eating into something like My Fitness Pal. But it's really dull having to do that for everything you're eating (especially as a lot of the carb and calorie counts can be a bit dubious)
Sorry - I realise that's not especially helpful!
I think if you keep things like pasta, rice, potatoes and bread out of your diet - and avoid most fruits, you'll probably be OK though. I'm guessing from your post that you're either vegan or vegetarian, so your carb counts will always be a little higher than if you were a carnivore.
There are a number of Fast800 groups on Facebook which could be worth joining, as there are a lot of pretty knowledgeable and experienced Fast800 people on there.