You should always use 2, 4, 8, 16, 32, etc pools. Always. This is because of the amount of 1 seeds, 2 seeds, so on, that make it out.
When you divide a bracket into sections, you want an equal distribution of seeds. So let's say you have a 48 man bracket, 16 pools, top three make it out. In both halves of the bracket you have eight 1 seeds, eight 2 seeds, and eight 3 seeds. In each fourth of the bracket you have four of each. In each eighth you have two, and in each sixteenth you have one. The same can be said for the two and three seeds, as well as the byes.
Now let's say you did, say, 6 pools, top eight make it out. That equals 48, right? Well, the problem is the distribution. You do have three one seeds in each half, but in each quarter, you don't have an equal distribution. You have six 1 seeds in two quarters, and in the other two quarters, you have 1. This is unbalanced. This isn't fair because you'll then have to decide which two 1 seeds are better than the others, to where they're in the quarter without another 1 seed. In place of where the 1 seed would be, you're going to have to put two seeds. Which is also not good.
What's worse is when people use an odd number of pools.
TL;DR
A Tier
2, 4, 8, 16, 32, 64, 128
B Tier
6, 12, 24, 48
C Tier
10, 18, 20, any other even numbers that haven't been mentioned
D Tier
Any odd number