If you distract your backers with too many different levels they'll get confused and just walk away. So keep it simple and limit your rewards to around five options.

Here is some amazing research on Kickstarter Tiers and total funding percentage done by Craig Mod. He gathered data for the top 30 grossing Kickstarter projects as of March 2010, then mapped out the number of pledges per tier, looking for a balance between number of pledges and overall percentage contribution of funds to the data set. Here is that data with the top five grossing tiers by percentage contribution (pay attention to the yellow rows when establishing your pricing).