Somewhere around 10 I'd say, and the pricing obviously depends on how much you're asking for. I think it would be hard to find anyone willing to pay over 500 on one kickstarter, so make sure you have a lot of rewards at low prices, and a few at high ones, because chances are if backers have the money to dish out on unknown kickstarters, they're usually not looking for the rewards.
Rewards will only get you so far though, what's important is that people are seeing your content!