A rice cake is 7 grams of carbs. If you’re going to use them for 3 hours and eat 84 grams of carbs per hour, that is 36 rice cakes you need. I also think they’re low in fructose, so you can’t absorb as many grams of carbs from a rice cake as from a more balanced source (which would have roughly the same amount of glucose and fructose).
A banana might have 30 grams of carbs, so we’re taking over 8 bananas.
I think you’re thinking of a different type of rice cake. These aren’t the discs of cardboard typically used as a diet food, but a squishy ball/block of rice - I’ve made them with sushi rice - filled with peanut butter/jam/filling of your choice.