You don't quite need to pledge more in order to receive additional character decks. In fact, there is only one pledge level in which character decks are sent directly to you, that being the Retail Edition. All other reward levels are Print-On-Demand. But let me break it down a bit further:
$15 - At this level and above, everyone receives a digital PDF version of all Crone content. Including the cards, manual, and any content unlocked by the kickstarter's stretch goals.
$20 - At this level and above, backers receive an at-cost, Print-on-Demand link for the base, 'retail' version of the game, which includes 124 cards and 4 character decks. By default these are the Fleshreaper, the Hedgewalker, the Shaman and the Stormkeeper, but as more stretch goal classes are created you may swap these out for whichever 4 classes you want.
$25 - At this level, in addition to the previous rewards mentioned, backers receive at-cost links to all stretch goals unlocked via the kickstarter, including (but not limited to) all the additional classes we are able to make.
$50 - At this level, backers receive everything mentioned previously. Also, we will physically ship them a 'retail' edition of the game.
Essentially, the $20 pledge level will give you print-on-demand access to the basics for playing the game: 4 classes of your choice, the manual, and the GM cards. If you're interested in accessing absolutely all of the characters, the $25 pledge will have your needs care of.