Eating too many onions may cause digestive issues such as bloating, gas pains due to the carbohydrates that onions contain.
But nothing serious will happen if you do eat too many onions other than the temporary digestive issues and gas pain or bloating like I mentioned above.
Onions are actually very healthy for you though and contain antioxidants, and onions can also help with weight loss and even help to keep your blood sugar in check.