The maximum that it will spend on any one vote is 5% of your voting power. You can only use 100% of your voting power in a 24 hour period. Imagine you start the day fresh with 100% voting power.
If you vote 10 times within the 24 hour period, you will use up 50% of your voting power. The other 50% would not have been used. This would technically be a 'waste', because you could have upvoted 10 more posts without affecting the weight of your other votes.
If you upvote 30 posts in the 24 hour period, then you will be over the limit of 20. I don't know exactly how the math works, but just for simplification sake - it would mean each of your votes only used about 3.3% of your voting power, which is less than the 5% that would have been used if you only voted on 20 posts.
Well that's why I asked for a clarification, to see how much do you know about this, you heard from someone that you should vote 20 times per day and you made a post about it? Well It's not true man, you can check your voting power Here. Now let me explain. What you heard was, that 20 votes in a 24hr period carry 100% voting power, if you vote more your power will go down, But it takes a lot of votes per day to bring that power down to say 40%. So you and me bringing our voting power down to 40% would't mean much, since we don't have high SP anyways, but this affects whales the most since they have high SP power, but In my experience whales do vote a lot!. Conclusion is: You should vote how much you want and the system encourages you to vote more, to be active and provides rewards for that. The 40% power is just my personal figure on how low I would go.