The question how much water you should drink during a race depends on 5 factors
1. Your weight
2. How much you sweat
3. The weather of the day
4. The distance
5. Level of experience
Weight and How much water to drink
The leaner you are the less water you require. If you have watched Kenyan runners run a marathon you might noticed they don’t drink a lot of water, it could be something to do with their lean bodies.
How much you sweat
Runners who happen to have a high rate of sweating will require to drink a lot of water to replace the lost water.
The weather of the day.
When the weather is hot, the rate of sweating is high and you get dehydrated faster. At the 2007 Chicago marathon the race had to be suspended after one runner died and many marathon runners taken to hospital suffering from heat exhaustion. The temperatures were abnormally high and runners complained that there wasn’t enough water stations.
The Distance of your run
The longer the distance the more water your body needs. Runners doing a 10,000 meters at a track are not provided with water but when distance rises to 21K organizers provide water.
Level of Experience
While elite runner like Kenenisa Bekele can do a mad rush at 10,000 metres without additional water on the way, the same cannot be said of fun runners doing a weekend 10K family fun day.
To know exactly how much water to drink, I recommend you take a sweat test.
The more you train, the fitter you become and you body is able to use water efficiently.
Keep in mind drinking too much water while running can be lethal, at worst too much water can kill, see my post dangers of drinking too much water.