I have 11 challenges and 22 votes so yes, I voted on a few challenges as well. The amount of tokens is relative and there is not a âgood numberâ because it will always be conditioned by the number of tokens other people use for voting.
As the reward is based on the % of your tokens out of the total on your side of the vote, you can never know what is the good number.
My suggestion if you want to maximize rewards is to use your tokens a few times but in the highest amount that allows you to keep doing things.
Example:
If you have 100K tokens I will not vote for more than 2-3 challenges to maximize the rewards. Take into account that it will cost you the same to vote for 1 Foam than vote for 100K foam and, if you donât want to loose money, you need to cover for at least the gas cost with your reward 
Hope it helps!