Answer: At 3.8 g of net carbs per serving, lemons are one of the few keto-compatible fruits.
Each serving of lemons (1 fruit, 2-1/8" diameter) contains 3.8 g of net carbs. That's a sizeable amount, but most people don't eat an entire lemon as part of their meal.
You'll often find lemons as part of keto recipes for their juice, which is fairly keto-friendly as well.