If you split the plate perfectly equally – measured by mass of the batter and mass of the onion, not by number of rings – then you should receive about 20% of the calories. Since the plate had 1500 Calories (if the plate had 1500 Calories), then you should receive about 300 Calories.
Like the above posters have mentioned, there are a number of variables.
It is actually quite possible, even likely, that the chef put more onion rings on the plate than 1500 Calories. If I were the chef, I would much rather add in a few extra rings than have a customer complain about not getting enough.
Did you split the rings according to mass or by size? Different rings will have different numbers of calories. On average, the larger rings will have more calories, so if you eat the 5 largest rings, then you would have eaten more calories than a person who ate the 5 smallest rings. Splitting according to mass will solve this problem, but is usually completely impractical since the rings would almost always need to be cut apart.
Assuming that there was in fact exactly 1500 Calories in the full plate, then on average one member of the group of five would eat exactly 1/5 of the Calories. But this is only on average. I would put the odds of you in particular eating exactly 300 Calories as so low that the chance is almost non-existent. You may have eaten more Calories or fewer Calories, but you did not eat 300 Calories.