Number of grains is a personal preference and in my opinion most people want lots of grains because they believe some of the false claims and opinions made by others. Ive owned bats with 7 grains, 8, 9,10,11,12,13,14 and my preference is 8-10 this is because in my experience ive had good performance and durability on bats like this. Ive had bats with heartwood butterfly marks specs etc and all i ask for is relatively straight clean grains as I'm not a fan of heartwood at all.
20 grain bat wont guarantee it will play better than a 10 grain bat but however a large number of people believe they do hence why its such a big deal when people buy bats.