How many candies do you get for every 2k, 5k, and 10k egg?
2 km eggs: 5 to 15 candy
5 km eggs: 10 to 21 candy
10 km eggs: 16 to 32 candy
Here is a Reddit Link supporting this claim.
(There was a single report of 4 candy from a 2 km egg but I'm guessing that was a typo/error.)
Each egg will always give you a random amount of candies between 3 and 35. The main difference between the 2KM, 5KM, and 10KM eggs is the possible pokemon that may hatch from them which you can see in this chart:
