I do statitics. Now imagine you want to it to be very likely for you to get at least a certain number of pillars in your first hand.
If you wanted at least a 95% chance of getting at least a certain number of pillars in your hand, this is the least number pillars you should put in a 30 card deck.
1: 13, 2: 19, 3: 24
Now say your feeling lucky and only want a 90% chance.
1: 11, 2: 17, 3: 23
Now say your feeling very unlucky and want a 99% Chance.
1: 17, 2: 23, 3: 26
And thats why most poeple use about 13 pillars. They might not relise it, but stastically it gives you over 95% chance of getting at least one pillar in a hand of 5 cards, in a deck of 30 cards.
I'd also recommend that you have a bare minimum of 11 pillars.
19 Pillars is a good number in a deck thats mana heavy.